Originally Posted by Andrew R Stewart

Another possibility is that some early MtB bikes used the "American" (these days I should really say "USA") 21.15mm stem diameters, a left over from decades of US bikes and Wald/Schwinn like products. (Ross was another who used this spec). With a simple caliper the measuring is easily drifted a bit from the actual. This is why afterr the thousand of assumptions and the one measurement, a trial fit tells all. Andy