I feel that the 2 frames above make good comparison, and I can see a constant in theior digits.
They are both made in Marvel factory.
They are both 1990 bikes, which is the 2nd and 3rd numbers.
The next number is the range in descending order. Eldridge was 3rd and Bear Valley was 4th. These numbers are clearly seen.
The rest of the numbers in the code are for size and factory production codes to recognise a date and individual responsible or manufacture. These are a little irrelevant for our purpose, and I feel it is the first digitd which recognise what frame you are looking at.
From seeing and talking with Marin owners and ATB sales, there were some exceptions to this rule, where frames were upgraded in the factory and appeared as another model in the showroom.
These basic rules of coding are applied to the later early 90's bikes in a very similar form.
Does anybody have any pre 1991 frames to compare with these other 2 which we are sure of?