NB: B18 distributors of the type that have the low tension connection made by an insulated bolt that goes through the distributor body (all B18 AFAIK) require a cap which has a small semi circular cut out to clear this fitting. Adjacent to that is a recess which allows the tab behind the retaining clip to locate the cap correctly i.e. Plug wire one will always be next to the notch in the rim of the distributor body. Therefore the caps for the B18 and B20 are different. The B20 condensor through connection is a much less annoying rubber block.
Although they look basically similar all Bosch rotors don't interchange. The main variable is the fit over the shaft. The wall thickness varies, thick or thin, which also alters the locating peg. A thin walled one may appear to fit but it will be loose. Result can be that it will destroy itself and the inside of the cap either immediately or a short but unpredictable time afterwards. Autojumblers should take a known "correct" type with them for comparison when looking at odd ones in trays and boxes. Caps with the notch for B18 distributors are rarely found now at autojumbles. Plenty of similar ones, so check carefully.
Thanks and well done Adam for the write up. Some useful information and extensive rebuild pictures of similar Bosch distributors can be found here.
http://www.glenn-ring.com/bosch/