This is a nice quality made Depth micrometer head with .001" graduation, and a standard thimble. It is marked "THE L.S. STARRETT CO. SPECIAL" on the thimble. The spindle size is .235" diameter, the smaller diameter next to the spindle is .100" for a distance of .5". Designed to fit Starrett standard micrometers or micrometer heads that have the .235" diameter spindle clearance hole in them. We tried this head in a No. 230 Starrett micrometer, and it will work, but is not the correct length (spindle) for correct calibration, so we advise, if you used it in a outside micrometer, to you will need to take the length into account. We gauged .63" when the micrometer showed a 1.00" reading. So, it is most likely designed to be used with a Starrett micrometer head mounting base. The head is in very good condition overall, no wear noted, no signs of use. Please take a few minutes to look for other machinist tools you may be looking for, as we have several hundred to choose from, both antique, and newer. We discount shipping on multi-item orders!