Yes the Canon 3.5/100 RF is the same design as the SLR lens of that era. Thanks for your comparison with the Tele-Elmarit. This was the smallest LEICA short telephoto, right? I know there are bigger Elmarits now - which may be better optically, but not in the "backtrousers league" which was the pristine audience of the Mountain-Elmar... amazing that LEICA now introduces a new folding 4/90 in that class...
About the performance of the CANON; I too was surprised of wide-open sharpness and contrast. This isn't to expect from a 50 year old design, in particular if it's a simple "triplet" design with 2 added lenses. I guess the Japanese designers wanted a lens usable unrestrained wide open when it's such a "slow" lens. But it can be said as well that it could improve more at f/8... a behavior like the modern C/V lenses.
For critical applications like landscape and architecture, the CANON 3.5/100 is equal with my C/V 2.5/75 and beats the "portrait lenses" Nikkor 2/85, Jupiter-9. It's on a par with the CANON 1.8/85, but at a smaller weight and fast handling. A very similar approach to the "Mountain Elmar class" is the C/V 3.5/90 "Apo" (really?). I haven't compared these two, but from readings I guess they are very similar as far as sharpness is concerned.
cheers, Frank