In the early 1980s, as the mountain bike industry was going through its infancy, the accepted standards were not yet in place. The 26x1.5" rim was widely used, but it wasn't quite universal. Some companies produced bikes with 24" rear wheels and 26" front wheels (Cannondale and Raleigh come to mind). There were a smattering of 650b-wheeled bikes, as well.
Some people had the desire to run more than one size wheel on the same bike, according to the conditions. They might want to run knobby mountain bike tires, one day, cyclocross tires, the next and road tires the day after. Getting all of these tires, and two different wheel sizes, to fit in the same frame was no problem. You can stick a 700c road wheel with 35c tires on it into almost any mass-produced mountain bike frame and fork from the 80s.
The problem is that the cantilever brakes won't work with both diameters of wheels.
Kent Eriksen solved that problem on the early Moots bikes, built in Steamboat Springs. He produced a product called Moots Mounts, which were cantilever mounts that attached to the seat stays or the fork legs, by means of a band clamp. You could loosen the clamp, move the mount to the correct position for whichever wheel you were running that day, then tighten it back down.
The Moots Mounts were available as an aftermarket item, so that you could convert road frames to cyclocross use, or avoid the mud buildup problems of below-the-chainstay U-Brakes, if your bike was so equipped.
I wish that Moots Mounts were still available. They go for obscene prices whenever they show up on eBay, so finding vintage units is not really an option. It seems to me that someone would produce something like them.
Some people say it wouldn't be worth the cost of tooling up for such a limited market. Yet, PAUL Components makes the Thumbies mount to convert bar-end shifters into thumb-shifters. That isn't a huge market, but it is apparently big enough to make it worthwhile not only for PAUL to tool up, but IRD has tooled up and produced their own version. Both of these sell for well over $50.00 a pair, and two different companies are finding enough buyers to make it a viable product.
I wish I had the connections in the industry to get this product built, again.