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.
That would explain the 1991 Bear Valley I had (Matte Zolatone/gloss black) that had Afterburner stays and a 1 1/8th headtube, sadly i sold that bike in 1992 and can't give you a frame number
EDIT: Just been perusing the 1991 catalogues, BV is supposed to have AB stays, but I had a 1991 BV with tapered stays as did a friend of mine.
Also messing with the serial numbers is the fact that (according to catalogue) BV and Palisades shared the same frame (Tange MTB throughout) and EG and PM shared same frame (Tange OS MT4), this would mean a model number in the serial could only refer to a frame type, not a paint/spec./model type, so a possible frame type list would be:
1. Team Issue - Prestige Concept
2. Team - Tange DB OS Prestige
3. Pine Mountain/Eldridge Grade - Tange DB OS 4MT
4. Bear Valley/Palisades Trail - Tange DB (MTB)
5. Muirwoods - Tange chromoly (front triangle only)
6. Bolinas Ridge - Tange chromoly (seat tube only)
Totally guessing these last two
7. Sausalito - Tange DB (MTB?) - 700c
8. Stinson - Tange chromoly (seat tube only) - 700c
Of course this is based on the frame numbers that WD-Pro and I have (1990 bikes) when applied to the 1991 range - very confusing I know and hard to confirm without more 1990 numbers and a 1990 catalogue.
So - clear as mud then
1986 RM Ritchey "Canapurna", 1991 Marin Eldridge Grade, 1990? Vitus Futural CX, 1995 TREK Singletrack 950, 1996 Muddy Fox Monarch Team, 1997 DMR Trailstar, 1993 Pro-Flex 853, 2001 Santa Cruz Chameleon