That is definitely a Super 55 serial number, should be a 1957 model year. It is a later, 'improved' Super, so it would have a 12 volt system with key start, a hydraulic system like the 550, along with other improvements over earlier Super 55s. 

Switching the radiator shell between a 550 and a Super 55 would be fairly simple, so yes, they could have been switched at some point in time.