Well done Stevie
sounds like it was indeed similar to what I was trying to describe- what I called a tiny grub screw with a pin on the end is what is actually called a key screw. And this then has a second retaining grub screw on top?
The collar as I called it appears to have a finely serrated gear on your mic, which is what drives the counter?
was the key screw damaged at all ? The tip of that component is very fragile and the one on my mic had been damaged which is why I replaced it. This might be why you find yours is not reading precisely. The profile of the top may not be sitting accurately in the groove as it might not extend down into the groove quite as far as it should.
if the tip has snapped off it will mean that the shoulder on the screw will rub on the spindle before the tip of the pin / key gets to where it should be within the groove.
These instruments are precision made and that part is really like a watch component. I suspect the pin/ key is designed to be sacrificial so it does not damage the spindle it keys into.
I think the damage occurs when someone tries to dismantle the assembly without first removing the key.
if yours is the slightest bit rough in use it may be worth getting and trying a new key from Mitutoyo.
with a quality instrument like that it is so nice to have it ‘spot on’!