Yes, it’s a MK 2 Nomad. If you’ re happy with disc brakes and can obtain a Mk2 Nomad disc fork it should be possible to fit 650b wheels. See the Mk2 Nomad, Pendix adapted, for sale here
https://www.sjscycles.co.uk/bikes/thorn-nomad-565l-with-pendix-motor-ex-test/
I could be mistaken but I’m sure this has been discussed on the forum and there might be just about sufficient vertical adjustment on some rim (V ?) brakes to accommodate 650b rim brake wheels. Such wheels aren’t (yet) common apparently but I read in Thorn literature that Thorns have had some made for them.
That Mk2 Nomad with Pendix proves that 650B/27.5/584 wheels will fit the Mk2 Nomad with disc brakes, and with clearance for 50 mm tyres. I'm less certain that it will be possible with rim brakes, there is a 12.5 mm difference on the radius between 584 and 26"/559.
When I moved to France in the early 1980's I had difficulty converting from the English standard 27"/630 to the French 700C/622 standard, which is only 4 mm difference on the radius. The rear brake was OK, but I had to get the front brake bosses on the fork moved and rebrazed.
Even if it were possible, I don't think it would be very practical on the Raven/Raven Tour frames as these have less tyre clearance than the Nomad.
Thorn currently list 5 different 650B/27.5/584 rims available for rim brakes, plus 2 others not in stock, so they appear to be getting more common.
