I have a lot of old SIG contrast sights and dead night sights from various manufacturers. I know the P220 generally takes #6 and #8. But I didn't keep track of what guns these came off of, though all were classic P-series SIGs. what measurements am I looking for with digital calipers for 6's and 8's? And where to do I measure?

This might help.

Awesome. Thanks.

I just measured them and there sure were a lot of oddball sizes in there and I'm sure they all came off of SIGs. But I found four rears that were definitely 8's and two fronts that were definitely 6's. The rest were all over the place.