[Geoff Kingston]

The 85 series engines A to C were found in the 400 and 401, the 100A in the 403 with a few late cars fitted with the 100B normally found in the 405.
The 100D2 was supplied to AC for the ace though some found their way into Bristols own cars, this is probably the most valuable/sought after of the road units.
The FNS series engines were supplied to Frazer Nash mostly for racing use and likewise the BS series engines were again mainly intended for racing use and sold to a number of customers for that purpose perhaps the most notable being cooper.
All of these numbers are the engine type numbers and all of 2 litre capacity. The 110 series engine that followed was of 2.2 Litre capacity and fitted to the 406 and some AC's. I am told it is not eligible for Historic Racing so is not so valuable but is a upgrade favoured for the earlier cars which is a shame as the 406 is a good and to some degree under rated car.
On the engine identification plate the numbers that follow the Type number as above are the actual engine number, it is the type number which will identify the original specification and you will sometimes see engines with plates stamped with for example 100A/B2 which means that it is a 100A engine modified to 100B2 spec rather than a unit originally built to 100B2 specification. In a lot of cases it was down to cam and carb changes and now if you have the money and or the skills virtually all of these units can be developed to provide more and properly done reliable extra power.
Most of the books on Bristol Cars have a section on the engine types and at least one of the Club websites also has a lot of information on the subject.
Geoff Dowdle who has also replied on this I am sure can give you a lot more information and help on these engines.