Any Idea What This Is?

Comparitor...You compare your part (that you want to measure) against a standard, either another part or a Jo block, etc. If youir part is supposed to be 2.3750, you set up Jo blocks to that size and set the dial indicator to zero. Back the Mike head off, put your part to be tested in place and turn the mike head to (what ever it was with the stack of Jo blocks in place). The Dial indicator will read zero if your part is correct.
 
I believe the original SuperMic is Pratt & Whitney; Starrett and Brown & Sharpe have the same arrangement and features. Likely they licensed the pattern to other manufacturers.
P & W's had the mic head built in and a linear indicator. One thing making these so accurate is an adjustable 'stage', a Z axis between spindle and anvil to insure round parts were positioned identically. The parallelism of spindle and anvil is so perfect, both flat and round gauges will record the same size.
 
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