Without going too far down the rabbit hole here...
As noted above, the earliest version (the Marshall G12 Vintage) was conceived for Marshall, specifically for use in the Studio 15 combos. The earliest ones I’ve seen were actually vented models with ‘85 date codes (the T3896/T3897). Marshall then started using them in special edition Jubilee cabs and the first 1960 AV/BV cabs starting in ‘87. The earliest Marshall 1960BV cabs had standard JCM 800 front tags with 1960 AV/BV serial plates with their own serial number scheme before the “Marshall Vintage” front tags were made.
As Marshall was using their proprietary model in the ‘80s, Celestion put out their own standalone version in ‘86-ish as well (T3903/T3904). Both were available into the early ‘90s when production changed. I’ve tried to pin down exactly when the change happened, but the closest I’ve gotten is in the ‘91 - ‘93 range. Mesa used the Celestion V30 as well, and weren’t happy that they changed the production on them. Upon request, they coopted the original T3903/T3904 as their own proprietary model (T4335/T4416). So, the Celestion V30s starting the early ‘90s were made somewhat differently, and the original V30 became Mesa’s proprietary model.
Marshall also changed the production of their G12 Vintage around this time, though Celestion will deny it
So going into the early ‘90s, you have the oldest Marshall G12 Vintage which are a little darker and woodier, Celestion’s original models, Marshall’s brighter and more upper mid heavy model that came about, and Mesa’s models which should be the same as the oldest Celestion versions.
If you hear any V30 on an album prior to ‘90 - ‘91, it’s either the original Marshall model or the original Celestion model, and if you have a Mesa T4336/T4416, it should in theory be the exact same as the Celestion V30 you would have found in the ‘80s. Where did those changes happen? No one really knows, and Celestion will deny quite a bit, but the odds from what I’ve seen are sometime in the ‘91 - ‘93 range.