Or just a German SSR. I don't see any reason for the Soviets to not outright claim that all of (former) Germany is a legitimate part of their union.
I haven't thought about the name of that SSR until today and decided for Labe as it was purely geographical and not ethnic as the original concept was to de-germanize Germany.
Prussian SSR would be an insult to the Saxons, Thuringians and Mecklenburgians in this region and not make things easier. Besides, the term "Prussia" was to be purged from the map after World War II even in OTL.
However, I am in ASB-country there anyways - so feel free to let anything happen.