Hi radin, to answer your many questions, the good news first:
Yes, the 22mm tube Kahles Mignon scopes, top in my photo, were available before WW1, the 26mm Heliavier, below, came about 1920.
End of the good news!
As I wrote before, the Wiener snap mounts were handcrafted one by one by individual gunsmithes. Not two are really alike and nothing is interchangeable! As they came with rings soldered to the scope's steel tube, you need a gunsmith able to completely disassemble the scope and resolder the "rings" if you want to adapt the mounts to a different base length. Such people are hard to find nowadays even in Europe. I have never seen clamping rings on these mounts.To further complicate the situation, on your Henry the bases are mounted the other way around than usually seen: as you may see in the above drawing and my photo, the "hook-under plate" was usually mounted at the rear, the "horns" for the snap fasteners at the front end. As the locking foot is usually fitted to the larger objective bell and the rear foot to the tube, it is impossible to simply exchange positions.
These mounts were never popular in Germany, Germans preferring claw mounts from the start, so you have to look in Austria. As the Austrians changed over to clawmounts (with different dimensions, of course!) in the 1930s, the Vienna snap mounts are completely obsolete and any spares have to be custom made by file, as the originals were.
If you ever find a scope+mount that fits your bases, it is probably the one that originally belonged to your rifle!