It can be a Leica mount, but it could be a Fed's too...
I bought several years ago a Tessar (s/n 271317, so made in the 1910s...) mounted on a fake Leica/true Fed S with such a mount. And this curious camera works perfectly, in spite of its cyclopean look!
About Peter's lens, I share David's opinion. It seems there were only 57 5cm LTM Sonnars made before the war (!!) and 2250 just after the war (s/n in the 28xxxxx), certainly at the request of the Soviet authorities. A very poor chance to put a Sonnar on a Leica...
Jacques.