[Ron Ramirez]

...in January 1932 the Model 551 Colonial Clock Clock used a model 51 chassis, but in June 1932 the same Colonial Clock used a model 52 Chassis. Why did the model numbers on the Chassis change?

Because Philco could...and they did.

It was probably purely for psychological reasons. The model 51 was "last year's model" by the time the new models came out in June 1932, so Philco's brass probably decided to just change the model number and otherwise retain the same chassis. The 52B and 52C did have new cabinets...

Philco did the same thing with model 36. The chassis is the same as model 35. The 36B cabinet is quite different from the 35 cathedral, which was identical with the 70 cathedral save for the exposed speaker screws in front.