Need some pics.
I have seen XS650 US Custom sold or advertised as an 80 and 81 but they looked exactly the same with mag disk brakes on the rear.
The Euro Standard with dual front disk brakes, (77-79), had the caliper mounting lugs lower on the front forks to accommodate the smaller disk. The US Standards, (77-79) and Specials, (78-83), had the larger front disk so the caliper mounts on the forks are higher up the leg. The smaller disk is the same as the rear disk on the Specials.
Again some pics would be nice.