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.