There have been many different systems to provide essentially “electronic sprockets” on audio tape to lock the tape, speed-wise, to film or video being shot at the same time. In the late 1960’s, three versions of time code were developed. One developed by EECO (1967), one developed by Siemens, and one developed at the Canadian Film Board by Leo H. O’Donnell. It was this last system (thank you to Corey Bailey for clarifying this) which was later standardized (I want to say the mid-1970’s, but that date is not certain), by the Society of Motion Picture and Television Engineers (SMPTE) and the European Broadcasting Union as SMPTE/EBU TimeCode. This became the best and most widely used system of locking differing transports together. Here are some links that might be a starting point:
Wikipedia Leitch Video (Canada/USA) Phil Rees (UK) Simon Fraser Univ (Canada)
Here is a drawing showing the dimensions of the centre track timecode track on 0.25 inch magnetic tape. The audio tracks are 82 mils and the timecode track is 31 mils. 
Going back in history, we have a variety of different systems that embedded tones on tape in order to provide a speed reference. The best summary of these old systems I have seen to date is in the EBU-3095 document (European Broadcasting Union). It is now available from the EBU here. The EBU developed a film code in 1985, but I do not know how widely it was used.
One of the later, and more-common versions of this is NeoPilot This Nagra-developed system has two out-of-polarity tracks within the audio track. It is somewhat problematic when reproducing on two-track heads as the pilot tone (60Hz) does not cancel out–another reason to have a full-track mono head assembly.
Filmmaker, ethnographer, photographer, and writer Rina Sherman, Ody Roos, and I collaborated on a better description of this process:
Maintaining sound-to-picture (lip) synchronization while filming in the field has been solved in a variety of ways over the years until film was replaced by video for documentaries. The basic concept is to use an AC signal whose frequency is directly related to the speed of the film camera and record that signal on the sound recorder. During playback, the machine’s speed is controlled by this signal as the audio is transferred to sprocketed magnetic film for editing. The challenge of linking the camera and the recorder was solved first by a wire between the two or sharing a common AC power source. Later, two matched crystal-controlled frequency generators were deployed, one for the camera and one for the recorder with various methods of linking start markers (Richard Hess: personal email correspondence 2016-05-26).
Recording the synchronizing signal without degrading the audio quality was solved in several ways over time. Pilote dates back to 1940 in Germany and the Pilot-tone invented in 1953 by Josef Schurer, development engineer at the Bayerischen Rundfunks, was an improvement on this system (Richard Hess 2006-2016). In 1954, Carsten Diercks, NWDR-Fernsehens operator from the ‘Hamburger Schule’, first experimented with it in a synchronous shoot for the film Musuri: Bericht einer Fernseh expedition nach Belgisch-Kongo (Carsten Diercks 2004 :32). In 1957, this Pilot-tone system was incorporated by Stefan Kudelski into the Nagra III tape recorder. From 1962 the Nagra machines were equipped with Kudelski’s Neopilot system (an update of the Pilot tone system) with its use of quartz crystal sync became the standard to synchronize these separate elements until the late 1980s. The main imperative however was to have a silent camera, once that was achieved, battery driven portable recorders were engineered by Perfectone with its auto-cancellation of the pilot signal, but a non-standard audio track. Kudelski (Nagra), Tandberg, and Stellavox ultimately all used the Neopilot system, which also had full cancellation of the pilot signal while maintaining a standard full-track audio recording (Ody Roos/Richard Hess: personal email correspondence May 2016). ***
From EBU T3095, these are the methods in the order presented in the document. The EBU has kindly made this document available as a PDF here.
Pilote (not described in EBU T3095) dates back to 1940 in Germany and it has an 18-mil gap at a 90-degree angle with no bias. Nagra’s Pilot-tone was an improvement on this system.
Pilot-tone 50/60 Hz from camera or crystal fed to centre track 90 degrees out of azimuth with the audio. Developed by Nagra and described in DIN 15575 (October, 1965). Pilottone starts when the last frame of film in the camera is exposed by a light inside the camera. This should be recoverable with standard timecode heads.
Perfectone uses a 4.0 mm play head in the centre of the tape with out-of-phase synchronizing signals at 2x the mains frequency recorded on the tape edges. 4-channel heads should recover this with tracks 2 and 3 summed for mono audio. Or the NAB 3-track cartridge heads could be used.
Neopilot is an update to the Pilot system and is also specified in DIN 15575. It uses two opposite polarity mains-frequency tracks. Each track is 0.45 mm wide and they are separated by 0.4 mm and this cluster is centred on the centre line of the tape. The audio is standard full-track. This requires a special head for recovery of the two out-of-polarity tracks.
Ranger (aka Rangertone) developed in 1949 uses a sync head at mains frequency 80 degrees out of azimuth with the audio in the centre of the full-track audio. This might be readable on a centre-track timecode head.
Telefunken this is a 0.8 mm centre track like centre-track timecode with a modulated tone. It is designed for NAB-standard (2.0 mm) stereo heads.
Synchrotone uses a 0.5 mm centre track with mains-frequency recorded on it. The centre-track timecode assembly should be able to extract this.
Leevers-Rich uses a modulated carrier either on one track of a 2-track NAB-standard head configuration (2.0 mm tracks) or a 0.6 mm centre-track. This system apparently used both 1 kHz and 400 Hz carriers, modulated with the supply mains or camera-motor locked pulses.
B.B.C. uses a 2.0 mm (NAB) two-track head with the mains signal on the lower channel.
Fairchild uses a full-track-width off-azimuth head recording an amplitude-modulated 14 kHz carrier. The modulation is at the mains/camera motor frequency. This will require significant modification to a standard head assembly to recover. It might be possible to use the record head, tilted, to acheive azimuth.
In addition, after this standard was published (I believe), Nagra came out with FM pilot for stereo machines. Synchrotone is similar, but I’m not sure if it’s identical with Nagra’s FM pilot.
In July, 1968, Loren L. Ryder published in the Journal of the Audio Engineering Society “Synchronous Sound for Motion Pictures.” In this paper, Ryder (of Ryder Sound in Hollywood) states that Neopilot accounts for 75% of all sync sound recordings made at that time. The remaining 25% in his experience were, at that time, dominated by the original Pilote system, the Ranger system, and a 2-track system (although he doesn’t seem to indicate whether it’s using a carrier or not).
I am aware of some usage of these mains-related frequencies being used on 4-track tapes for speed control. Both 1/4-inch and 1/2-inch tapes were used in this mode.
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*** From a book to be published in the fall 2017: Dans le sillage de Jean Rouch–D’après une idée de Rina Sherman
Préface Jean-Claude Carrière
Avant-propos Edgar Morin, avec Rina Sherman
Introduction Jean-Pierre Dozon
Éditions de la Maison des sciences de l’homme, Paris