It's a mystery!!!!
I have an older piece of equipment in my machine shop that needs adjusted for calibration, (it is .001 shy of it's zero) and I have no clue as to how it is calibrated.
It is a Lufkin inside mic 1.5" to 32" set with the number 681 on the barrel of the larger handle (I assume this is a model number)
There is a hole on the knurled area but it has been 'pinned' so can not access it. I have had it apart four times now seeing if I have missed anything and I even took it to one of my ASQ friends that is a calibration tech. and he had no idea either.
If anyone knows how to adjust this, has instructions from a similar set, or just plain advice because you have done it before please don't hesitate to send me anything you have. It is a nice set and in great condition.
Thanks a lot! Ree