This is a compact 4/4 rotary four valve C tuba. It's fun and plays easily. It's in tune with itself. I play the open 5th partial (E) with 1-2, but Eb is fine with either 2 or 2-3 (like a 186).
Robert Tucci graciously looked at a few pictures, recalled some memories of the 1960s and '70s, and said his opinion was that it seems to be "a genuine Scherzer instrument, not something put together from available parts." Also, this:
Your instrument looks like something made in the mid- to late Sixties. Fred Marrch, the deceased former owner of the old Custom Music Company, was one of the first American dealer/buyers who worked into Europe. At one time or other, Fred was imporing "Alexander" and also genuine "Scherzer" tubas. The Scherzer instruments were all either one-offs or in very small quantities.
Mechanically it's in great condition. It used to have S-links that have been replace by sturdy DuBro connectors. The valves are light and fast. The slides all move easily. I had the main tuning slide extended a little since I was always pulling it out to the limit, and I'll include the old slide tubes.
There are little scratches and some nicks and tiny dents. If it's been refinished, it was a long time ago.
Here's the sale listing where I bought it in 2020. That thread has discussion about how nice an instrument it must be, though it's a little hard to be sure if everyone was talking about the same instrument, since "Scherzer" and variations on "Sander" seem to have appeared in many places. After I got it, I thought it mysterious enough that there's also this thread with guesses about the instrument's origin.