You can see them fairly often on e bay, called the #246 "Ultra Mike" Micrometer / Comparator; cost $495 in 1970 according to my Brown & Sharpe STM-5 catalog. 0-4" range direct reading to one tenth of a thou. and comparative measurements to 50 millionths. If equipped with an electronic gage head instead of the dial indicator, comparative measurements to ten millionths.
We've got them on anything big/expensive enough to be considered an accounted capital expense at work. Basically a tag with an ownership stamp (eg property of company blah) and a serial number for accounting to keep track of where the item is and when it enters and leaves service. Ours are usually stamped with a reminder to turn the tag in if the asset is scrapped or not to scrap the asset without notifying accounting
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.