This is an unusual 6 Valve F  Viennese tuba beautifully inscribed:

Scherzer was born in 1895 in Markneukirchen and died in 1962. He made brass instruments in Augsburg from 1930-1955.  There is a further inscription: Bayr. Ilants Theater, Inne No 11 (Not sure of meaning!).

To figure out the  fingerings for a 6 valve F tuba,  the upper three valves, on the left side of the instrument are "1 - 2 - 3" as commonly found on most tubas. On the lower right, the third (sixth) valve is 2 1/2 tones, effectively a "fourth valve". The fifth valve is a compensated half-tone, relative to the sixth. That is, low "C" is played on the sixth valve, low B natural on five and six. The first / fourth valve on the right is the same as the regular third valve, one and a half tones.  This will give you a "D" as with the first and second, or third valve.  Or a Db, using the fourth and fifth valve. Depending on the tuba, this can be a bit sharp. In practical terms, one plays low C with the sixth, low B with the fifth and six. low Bb with the fourth and sixth and low A with the fouth, fifth and sixth valve.

As for the 2-3 combination, you have one on the left and one on the right (valves four and five). Some of the former "Sechs ventilige F Orchestertuba" artists would tune the one 2-3 combination high (left valves) and the other (right valves) low.

Bob Tucci once told me that back in his Vienna days (1962-66) he played one of these, a "Dehmel", which was really difficult to play - and with terrible intonation. It seems Arnold Jacobs also had one of these, a present from Fritz Reiner, which untimately landed in the middle of Lake Michigan! Apparently there were few good 6 valve Viennese tubas, relatively speaking, and even fewer people who could play them well!