From what I've been able to determine, here's a summary of the right clutch cover versions. The part number suffixes will likely match the casting suffixes, for at least the first several years.
256-15421-00
Early '70 XS1, up to s/n S650-001124.
Uses o-ring on side filter cover.
Side filter bolts are somewhat vertical, about 11:00 and 5:00.
256-15421-01
Later '70 XS1, starting at s/n S650-001125, '71 XS1B.
Uses gasket on side filter cover.
Side filter bolts are somewhat vertical, about 11:00 and 5:00.
256-15421-02
'72-'75 XS2, TX650, TX650A, XS650B
Uses gasket on side filter cover.
Side filter bolts are somewhat horizontal, about 10:00 and 4:00.
256-15421-03
'76-'79 XS650C, XS650D, XS650E, XS650F
Uses gasket on side filter cover.
Side filter bolts are somewhat horizontal, about 10:00 and 4:00.
These later versions, I'm not quite clear.
256-15421-04
4E3-15421-00-00
99999-01146-00
'79-'83 XS650F, XS650G, XS650H, XS650J, XS650K
Uses gasket on side filter cover.
Has oil sight glass.
Side filter bolts are somewhat horizontal, about 10:00 and 4:00.
At some unknown (for now) version, the label "Kickstart" (?) was added above the shaft boss.
From what I've been able to determine, all covers are interchangeable. However, there was a change to the engine cases mid '70 XS1, at s/n S650-003430. Cases before that s/n used shorter cover screws...