I asked a manufacturing tech Professor back in school about how they might have done it. The examples I have seen used endgrain balsa, which makes sense as the balsa is great in compression that way. We concluded they were rolled closed after the balsa was inside. the tricks we though about were:
How was the rim secured closed, we could see the pins or rivet heads on the glue side, the other side is hidden by a transfer.
How did they keep the inside surface highly polished, no plastic film to protect back then. Leather rolling wheel? Or just lots of labor?
We figured the work hardening of the alloy got the strength up.
The tooling must have been pretty flexible, Scheeren rims were made in a number of diameters, from "16"" for Moultons, 20", 24" and of course 700c.
As a Junior (schoolboy) racer, I liked them as they were when polished just like chrome under the night velodrome lights.
As to how the Weltmeister rims got so light.. well anyone have a broken one?
Anyone know the last year of production? I have seen them "new" as late as 1971, but they could have been NOS by that time.
John Jorgensen
Torrance California USA