There were differences in the Marin frames, however I believe some frames were shared across several models. I think the Pine Mountain and Eldridge Grade had the same frame, but in different colours and with different components.
The frames all looked very similar, same braze-ons and drop outs, but were made with different quality tube sets, for example in 1991:
Team Issue: Tange Prestige Concept
Team: Tange DB Oversise Prestige
Pine Mountain: Tange DB Oversise 4 MT
Eldridge Grade: Tange DB Oversise 4 MT
Bear Valley: Tange Double Butted CroMo
See Here: http://www.retrobike.co.uk/gallery2/d/12560-2/1991.pdf
I have a 1990 Pine Mountain, with the thin rear stays, rather than the later, fatter "afterburner" stays. At about 5.5lbs it's not a particularly light for a steel frame, which can be under 4lbs, but it rides nicely and personally I don't think a pound or so either way makes much real world difference on a complete bike whereas overall handling and feel are instantly noticeable.