Sure, a couple one off prototypes. There were all sorts of odd things early on. I also recall some dude that rode around a homemade 26 x 1 3/8" based 'adventure' type bike, but it would be a mistake to say that ISO 590/EA3 rims and tires were ever an MTB thing.

Anyway there's no point in splitting hairs over semantics. In the many thousands of MTBs I repaired or built up in the 80s, not a single one had 650B tires. So yeah, technically there were two, but realistically and certainly as a production thing, they did not exist.

Among the more hipster mechanics, at least in the SF Bay Area, 650B always had this unicorn and super cool quality. It was clearly because of the extreme rarity. Talked about but never seen. S6 was rare, 650B only existed on paper. I never saw one in real life till one of my friends brought an old Herse back from France in the late 80s.

The OP has disappeared, and left no pictures. We may never know what he/she has. So this is completely academic.

In my years of wrenching on thousands of bikes, I've never laid hands on a pre-1990 650B MTB either. So I agree that they were not a a big thing. But that was never my contention, in fact I was saying the opposite. Even if we call the production 650B Raleigh mountain tourer a "mountain bike" (which I have no problem with) such a bike would still be very rare.

I'm not going to argue whether they didn't or don't exist. That's settled. "Technically," we know of at least two customs, we can with some confidence assume more than that were made. We know of one production model, and there were probably a few more that may not exist in the USA. So if someone found a pre-1990 650B off-road bike, and wanted to hack it up to take 29er wheels, I would recommend against it, as it would be an "odd thing," and most likely worth preserving.

I think we have been trolled.

I'm surprised it doesn't happen more often.......

Yeah, we are in agreement on the actual facts. There's no point in worrying about interpretation.

But, as long as we are on this, we should all remember that in France, 650B really was the off road adventure bike standard size. I'm not sure I'd call those old rando/touring bikes MTB's, but they were designed for gravel road riding. Credit where credit is due.

Rivendell played around with 650b pretty early didnt they?

So this is all the information I needed. I had no idea what 650b was.

...converting a 26" vintage mountain bike to a 29er. Awkward.