This is a model M-S-B. If you read my write-up on the Model M-S, you'll notice how much different this looks from the M-S. It's a completely re-designed unit. It doesn't need batteries and it's in a much smaller, bakelite case.

The difference between this and the M-S-A is the meter face. The -A had a black face with white lettering. This reverses it to black on white, which I think is much easier to read. It also added a red scale (hard to see here) so you can direct read Kodachrome (at the time, Kodachrome only came in one speed: ASA 10).

Just as a note about Rhamstine meters: one of the things that makes it extra hard to research these is that there are no patent numbers. Every Rhamstine I've seen is "patent pending," from the earliest I own (the M-S) to the last (a 14-A). They may have actually filed patent applications but I can't find them. Yet Weston, General Electric, and other contemporaries all managed it. Make of that what you will.