I am usually reluctant to describe a camera as a prototype unless I have a pretty good reason to believe it truly is. So many "prototypes" are cameras that have been modified after the fact by repairmen or hobbyists for one reason or another. The fact that this camera appeared in Portland, the city where Stereocraft and Viewmaster were founded, the number of small differences in construction and finish, plus a low serial number of 1050 leads me to believe this is a pre-production camera, and not one that has been modified. In addition, a camera of similar finish can be found at Viewmaster Resource .
I suspect that a small batch of cameras were made and tested, and the improvements were incorporated into the production model. One has to wonder what kind of internal modifications were made as well.
From the front, aside from the obvious difference in the finish on the top, the switch on the front is marked "1" and "2" on the early camera, and "A" and "B" on the later. The later camera illustrated here is a fairly early camera with a serial number just under 10,000.
The film counters are marked slightly different and the top of the wind knob is plain on the early camera.
The earlier camera is shown on top. The directions are different between the two, otherwise the bottoms are the same.
The latches are different, the later version being more complex, but also more secure.
The cameras with the backs open. The earlier camera has a plate covering the drive spool. The outside backs of the camera appear to be identical.
The case on top came with the prototype, and appears more crude than the production case shown below.