Is that Wald stem the one that is made from stamped steel? If so, I would not get it for even teh lightest riding. Much less on a MTB.

There was a time in the early days of MTB when a number of bikes used that size. And Schwinn, and a few others, used them on mid-line road bikes also in the early 80's. So, there are some floating around out there. If you are patient and keep an eye on eBay, they do turn up. But, its hard to find one with any kind of reach at all. For some reason they always seem to be made for T-Rex people.

Over in C&V world, a few people have toyed with the idea of turning a smidge off a normal stem to get one to fit. But I am not sure anyone has actually done it. It strikes me that it would be somewhat tricky machining one down. And sanding one down would take a loooong time, I think.

If you have found an adapter, then I would jump at it. But is it really for that size? Maybe someplace in the BMX world someone has a use for that adapter, so I suppose it could exist. But don't know.

So, I guess I have no particular helpful info you, other than the fact that others have faced this problem before. Good luck.

jim