Very interesting to see these rather unknown shorts discussed here. I came around reading this thread only now, as I simply didn't have enough time in the past weeks. I enjoyed reading your comments wpqx, and I'd like to read more. Usually this is a neglected area, probably because of the simple reason that there weren't many feature films that dealt with similar ideas. To this day, short films continue to be ignored.
By chance I also stumbled upon some avant-garde films, two or three months ago when i was watching a lot of (mostly animated) silent shorts. It was a VHS with six early works of Hans Richter, and I also saw the two films you mention, Rhythmus 21 (1921) and Vormittagsspuk (1928) . Can't add much, as Richters films are very abstract, and he seems to be interested in transformation through movement.
I must say that I enjoyed all of them, but didn't find them groundbreaking or remarkable. The exception could be Rhythmus 21, which was probably something new for its time, and which i incidentally also enjoyed the most.
Hmm, I recall that I have taped some experimental films from TV some months ago. Maybe I'll write something if i find them.
Btw, Uberfall (1928) sounds very interesting. Could you write a bit more about it wpqx? Is the story told in real time, or do get several perspectives through "timeshifts". How long is the film? I haven't heard of Metzer before...