I had wondered the same and did some research but I can’t provide an answer. However here is some objective, potentially relevant information which I can provide.
Jumbo Carddass
Jumbo Carddass shares some of the Part 3/4 art and was released in December 1996 so we can set an upper bound on at least some of the art to before that point. Note that Carddass part 3/4 were released later, in April and June of 1997 respectively. Jumbo Carddass has really early variants like Charizard with only one horn, so that speaks to how ancient some of these arts are. Pkmn Red/Green sprites have two horns, so either that Charizard art was made before the red/green sprites, or else there was some span of time with inconsistent artwork. However, it’s not possible that all of the carddass 3/4 art was made before RG sprites because it is known that many pokemon were first depicted through the creation of their sprites.
Individual Pokemon - Early / Quirky Variants:
Kakuna
Kakuna has arms in Carddass Part 3/4. After a certain point, Kakuna never had arms anymore. The latest source I could find with arms besides 3/4 is its pokemon blue sprite. Link for all japanese green (left) vs japanese blue (right) sprites.
Pikachu
The most obvious one, pika has a white belly here. White belly is shown only in early art, and you might alternatively call it “distinctly colored belly” rather than white because that’s how it appears in the early game sprites. Sometimes there are edge-cases with a whiteish belly but no clear border between belly and body, like in the Fossil card Mr. Fuji, but distinct-border pika I’ve only seen in earlier releases.
The earliest appearance of yellow-belly I have seen is in the March 1996 corocoro magazine (released 2.15 1996) where you see the standard RG sugimori art. That’s shortly before the release of the first games.
The latest appearance of white-belly I have seen besides Carddass 3/4 is in the release of Pokemon 4Koma DX, released August 1996 (link). But I have no guarantee that my search has been exhaustive. Note: @firebirder31 pointed out nicely that detective pikachu has a white belly so it breaks this rule. If there’s any other example of a distinct belly that’d be interesting to document.
Slowpoke
Another quirk, slowpoke has a white belly in the Carddass 3/4 Aerodactyl card. Usually only slowbro has that belly, bro.
Others
There are yet other differences like charmander’s back spikes, but these existed for a while in the early days of pokemon so it can’t be used to date the 3/4 artwork before other things.
The Big Question
Could this be the earliest pokemon artwork? A lot of it has an earlier, more “beta” feel than the stock red-green sugimori art, which is the only competitor for a full-151 art set. I guess it seems doubtful because of the release date. But there’s no conclusive evidence either way so until we know more, who knows. A possible next step to learn more could be to try to get in touch with Sugimori or some of the other early pokemon people to see if they remember.