Need a bit of help. I have a 1983 XLX Sportster and I need to replace the rear master cylinder sight glass as the old sight glass is bugged. I have been looking for a parts number but all the parts manuals that I can find don't show  a sight glass in the master cylinder. The 1983 XLX did come out with a sight glass in the master cylinder as I bought one band new and pictures of the bike clearly show the that it had a sight glass. The sight glass is different from the one in the front master cylinder and will not fit properly. Trying to keep the bike as original as possible so don't want to replace the master cylinder with a new one. Does anyone know the part number or where I can purchase a new sight glass. Thanks in advance.

You can get generic replacement sight glasses off ebay.  They usually come in a couple of difference sizes.  I think 12mm and 18mm from memory.  Measure yours and do a search on there.