It kinda looks like a modern free-floater, but it's a single-tension design since it doesn't appear that the batter and resonant heads can be tuned independently. In that respect, it's pretty similar mechanically to early 20th century marching drums or student drums. A whole bunch of early 1900s bass drums were single-tension drums, too.I didn't spend too much time looking through catalogs on the Sonor Museum web site, but maybe it's a slightly newer relative of the D409 snare drum in the the 1957-58 catalog?
these single tension drums are/were designed to use corresponding heads. they have been and still are very popular in Germany. in the past Sonor had them commonly but some in later years were probably made by someone else and badged as Sonor. Lefima, Tromsa,Trowa, Tacton all made them routinely(Trixon never did) and I still see new ones branded with a name that I do not recognize(sounds German but probably Chinese). corresponding heads allow accurate tuning. i've got a few of these(dating from 1900-1970) and they seem to do best with fairly thin heads----diplomat batter top and bottom work well. ambassador and then emperor give an increasingly throatier sound. they are best with fairly light calf heads.
