It is a 3-point micrometer for diameters of about 3-1/2" OD. I thought is was an engine piston measuring micrometer but have been unable to find any reference to it and am hoping a keen eye on this forum will identify it and hazard a guess as to its value.

I don't know what that micrometer was used for. But what I do know:

Micrometers with V-shaped anvils are (most often) used for measuring items with an odd number of vanes, or fins, or flutes, or whatever. For instance think of a shaft with three equally-spaced flats running the full length of it. You couldn't measure the shaft diameter with a regular mic, you would need a V-angle mic with anvils set at 60 degrees from one another. This is the most common V-angle mic.

The anvils on your mic appear to be set at 108 degrees from each other, which would be for measuring a 5-sided item. Perhaps a small fan rotor blade or the like? Hmmm.

Plyerman, thanks for your info but it does not appear to apply to my mic. If you enlarge the photo, you will see that the two anvils are machined flat rectangular surfaces which are spaced about 50º apart and might be very slightly concave. There are also three machined circular flat pads at the bottom of the frame which would ensure the measured item lies flat and perpendicular to the anvils. To me, this mic appears to have been designed to measure concentricity of a cylindrical object such as an engine piston but that could have been done with a standard outside mic. The underside of the mic is not a flat surface which would preclude it from being a bench top tool, thus leaning more toward a handheld tool. There is very little wear on the painted surface which leads me to believe it was not needed on a regular basis such as on a production line. So, still an unanswered question for the measurement devices authorities.

Slocum, like all the major measuring tools companies, would have had a 'custom' or 'special', department.
This is probably not a catalog item but a special made for some specific manufacturing application.
The exact application may be nearly impossible to determine.

I agree with Plyerman that it is a V-mic but with a twist. If you place your bevel gauge on one flat and the blade on the other flat to get the degree you will know how many flutes it was made for.  3 flute is 60 degrees 5 is 108 and 7 is 128. My guess is a 7 flute.

The 3 flats I'm guessing is to be able to square the tool against a face on the part making it easier, faster and more accurate to measure the part and that it is a special tool for a specific part.

I am pretty sure it is a special tool but if you measured the range  E.g. 3"-3 1/2" and determined the number of flutes you could look up what a used 3"-3 1/2" V-mic ?-flute is selling for and get some idea on the value.