[Arran]

The model 51 and 52 are identical mechanically, I'm not sure what may have been changed between the two chassis but one will very easily bolt into the cabinet of the other without drilling any extra holes. Philco did something similar with the model 21, it used the same chassis as a model 20, but the same cabinet as the later model 70, but the control layout was slightly different, as Mike (Morzh) can tell you the model 20s are a TRF, not as good a model 70, but not terrible either. Philco had it's own cabinet shop so it wasn't unusual for them to do things like use up left over chassis, in new cabinets, before starting a new production run, in this case they didn't have to do anything special. It may have been a factory job, or someone had an empty cabinet that they wanted to use later on, but it's conjecture on my part, either way. I can't swear that it is an RCA speaker, but that welded rear bracket was a sort of trademark with their speakers, and the basket looks like something from an RCA (or CGE) set from the 1937-38 period.
Regards
Arran