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In the “early days”, or back when synthesizers where first being manufactured, different companies used different standards for controlling the synths.

As Lucia mentioned above in an earlier reply, how different companies early on used different system interfaces...

These incompatibilities lead Dave Smith Of Sequential to basically draft the specifications for MIDI (Musical Instument Digital Interface), that was eventually adopted by all the synth manufacturers.

Back to the old days...

Different manufacturers (typically in the USA) used the 1 volt/octave interfaces and trigger signals, usually Moog, ARP, Sequential, and Oberheim, as well as others like PAIA and Antares. However, some used “S” or Switch triggers, while others used “V” or Voltage triggers.

Most of the Japanese companies like Yamaha and Korg where using hertz/octave for tuning, but at this point (it has been a few decades), I do not remember for certain, but I suspect they used the the opposite of what Moog were using. At one point I had a tiny interface a friend made that allowed me to tack trigger signals from my Moog Source and Moog Multimoog synths and interface with my original Korg MS-20.

The best thing to do for a bit of history is to read through this article on Wikipedia - https://en.m.wikipedia.org/wiki/Analog_synthesizer