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Join contest SubscribeThey are mic balls...they attach to the thimble and anvil to be used to measure concave surfaces at the lowest point. The user has to calculate the diameters of the balls when reading the mic. They come is different sizes for different sized mic faces too and different manufacturers have different styles. These appear to be Starret brand, because of the metal, snap on housing. Mitutoyo have a rubber style attachment.
I bought Interapid, Mitituyo, & Browne&Sharp test indicators when I was tooling up after my 1st lathe purchase. The B&S BestTest are far & away my favorites. Looks like you got a nice one....