Now this is something you don't see every day. At first glance this looks like a red dial variant of the British Master III. But it's not! It's a Master II. The back reveals a blank factory-fitted plate with the number P2544 stamped on it. This almost certainly identifies it as a Prototype. Why it was constructed and just how many were made is not known. At the Sangamo factory they may have wished to do a practical evaluation of the new re-dressed red dial prior to putting it into production on the Master III. Note the Exposure Value scale, this being the first dial (on the Master III) to implement it - on both the US and UK Master IIIs. 

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What a find! I am eternally grateful to David Paschke who spotted it and bought it for me.