I'm gearing up to make a run of 70 degree locking pieces for people that like to make the little K guns. I have my grinding fixture done, material arrived this morning.
A little background on what I have done in the past. I make a fixture that holds the finish ground locking pieces at the necessary angle and grind the shoulder angle to a specific distance from the top of the fixture, then I flip the LP over and grind the other shoulder angle. This is the last step in the mfg. process.
For quality control purposes I measure the gap in a rifle using a factory LP then switch the LP out for mine. Based on the difference in the gap I can adjust my grind height so that my LP matches the factory LP inside the rifle. Once I determine the height, I mark the fixture for future reference. A few ten-thousandths of an inch on each shoulder can vary the gap length 2-3 thousandths on an LP40. This was shown through all the measuring in the LP type study and is why just changing one LP out for another usually affects gap.
Problem with my quality control check above is that I don't have an HK, much less a C93 to check them in the first time. I actually thought about buying a C93 but frankly I doubt that I'll sell enough of these things to make it a worthwhile investment. I thought about advertising for a "bad" one that needs work just to keep initial cost down.
I have enough loose parts that I could put together something to check the gap outside of the receiver, I just don't think it would give repeatable results.