Standard seat 74-79; The profile of the XS650 seat changed in the later years, but 74-76 was the same, (as this seat looks to be), and then 77D/78E seat was thicker padded and the 79F had a definite kick up for the passenger part of the seat. All have the same pan.
The Seat will fit all Standard models, 74 - 79.
Will fit the Specials, (78SE, 79SF, 79 SII, 80G SII), with some slight mods.
http://www.xs650.com/threads/standard-vs-special-vs-special-ii-seats.22157/