I've got a '75 B project which I'm slowly collecting parts for, and like yours it's a US model with the single large disc.Grewth, you’re spot on. I bought the bike because the black 1975 B model was one I wanted. Coming from the US it has no rot issues and this one is really low mileage. But because I like tinkering I just thought it might be easy to bolt on another disk and calliper…. I’m now inclined to leave well alone and just enjoy it for what it is.
In the US, the '75 B model was the sixth model in the XS series, proceeded by the two XS1's, the XS2, the TX650, and finally the heavily reworked TX650A.
Now here in the UK, we got nothing for '70 and '71, the XS2 was the first four stroke Yamaha to make it here.
Then for '73 we got...errrr.... the XS2 some more, but with bigger indicators I think.
Then for '74 we got ......more of the XS2 again, I don't think even the colour scheme changed from '72 although it was different from the US colour scheme.
In '75 everything changed with the appearance of the XS650B.
Not only did we get a completely reworked 650, virtually everything was new on the rolling chassis, and Yamaha even treated us to twin front discs.
Only Suzukis GT750 and Hondas Goldwing had twin discs up front then, even the mighty Z1 still only had a single disc.
I'm planning to root out an original twin disc set up for mine, the hard bit will be finding the UK type fork sliders for a '75.
The parts must be out there somewhere, they sold enough bikes back in the day.
Once you get the forks sorted, everything else is just standard mid '70's RD/XS parts.
And then I want to find the UK spec tail lamp and mount, like the one on a FS1E