One real thing for sure , all old time machinist who served apprenticeship know how to use and the many ways it should be used even today. Anyone who uses a mill , a planer , a shaper, and yes a lathe too. Surface gage is so underutilized now . In my ancient youth it was a required tool in minimum of one . That had to be the mid size models 9" + 10" rods , usually came with two back then. Smaller ones for smaller jobs bigger for big of course. I happen to own several one is to me Hugh weighs about ten pounds or so. The rods are 18" + 24" , I used it on many set ups on planners and shapers , back in the seventies . Sure don't seem like that's old to me but it is . With the pins down it could set up a casting with in .020 easily . Add an indicator and 1/2 thou is more then possible for squareness on the table.