The YZ-C fork caps and most XT/TT500 for caps interchange with the 77-82 PE forks (not 82 175 though). On some of the yamaha ones and early PE ones the thread/o-ring areas are different heights like Brent said but theres usually enough for them to work. Ive mixed and matched all sorts of caps around. Basically there are at least 2 yamaha 36mm type caps and 2 suzuki ones and they have the same threads but a bit different in lengths. You have to watch out for the early 80's 36mm IT forks as they are a different thread pitch though. Im not sure what the 82-84 PE 175 caps are but i would assume they will interhcange with some 38mm caps off a Yamaha aswell.
Ok ive just remembered what the thread issue is. You cant really use 78/79 PE, SP, DR-S or TS250 fork caps on the 80-82 PE forks but you can swap them the otherway around and use 80-82 36mm caps on the 78/79 PE, SP, DR-S and TS250 forks (although there is the height issue, so im more talking about aftermarket ones or if you were using different triples)