Pot codes... yeah, well... it's a bit of a black art, innit?
I'll share what I know... or rather what I
think I know. It's basically all theories in the classic scientific sense, i e interpretations that seem to work and have yet to be disproven. Proof would be credible period documentation spelling it all out, but I've yet to see that, and I doubt we'll ever see that, frankly. I mean, we don't even know who the manufacturers were in most cases.
Most of what I have is based on the guitars that interest me the most, i e Greco Fender copies. Which is a good thing, since they more or less invariably have a plaintext month and year of manufacture in their serials, giving you a solid date to compare any pot markings to.
Anyway, first up:
The "YM X" codes seems to have been first interpreted by Burny enthusiasts, as an aid to dating unserialled instruments from the 80's and 90's. It also works fine with Grecos, which have them from about 1980. The earliest I've seen in Tokais have been 1982, and they seem to be in most guitars from about 1985-86 or so.
It's on pots with embossed markings, below the kOhm number (rarely, there's what appears to be a parts number instead, I've seen 504071, 504041, 50472 and 50442 so far).
The first character is the last digit of the year, the second one is the month (1 - 0, plus N for November and D for December). A space follows, then a letter, almost always K, but P turns up infrequently and I've heard V being reported as well. The meaning of the letter is unknown. Some sources claim that P is on push-pull pots, but this is clearly incorrect, since PP-pots are technically very different, with a large switch box (like the lower part of a mini toggle switch) attached to the bottom.
You see these in various guitars from about 1980 to at least 93-94, so there's an overlap. So, for instance, a 2 for the year could mean either 1982 or '92. In practice, that's seldom a problem, most guitars have construction tells that lets you tell the decades apart.
Late '81/early '82 Burny RLG-50. Visible codes are 18 for Aug '81 and 1N for Nov '81.
1985 Tokai TST-50, 56 for June '85.
Tokai PB-45, 26 for Feb 1982, with the very rare P code (and clearly no push-pull pots). This is one of the earliest instances of pots with these codes I've seen in a Tokai.
0D for Dec 1980 in a early'81 Greco SE. The D is easily mistaken for 0.
The "parts numbers" version of this code style. 504071 and -41 with 06 and 07 date codes in a 1990 Greco RG-950 Rickenbacker 360 copy.