Scooterotica

Sorry to ask probably an easy question to most BUT...what is the origin/difference between a casing with the number 1991 0002 and 1991 0020 ?? (apart from the fact the 2's are in a different place please!!! ) . The latter number is listed in my parts book as correct for the series 3 range, the series 2 is listed as 1901 0020 and there is the web difference I'm aware of.......I've come across this a few times on other items but not bothered to find out until now....Thanks

Early casings (not sure how early) had 6mm studs for the exhaust instead of the later 8mm, that would make the chaincase cover different too, maybe it has something to do with that. I've got parts books for S1, S2 & S3 and they all list the cover as 20 and not 02, but as the books are modern reproductions they may have been udated with the later numbers.