The Davidson Seating Depth Checkers attach to the blade of your calipers with a small thumb screw and consists of a nose piece for the bullet and a base piece for the case head (sold separately). These two pieces together serve to align the cartridge for accurate measurements. The nose pieces can also be used without the base pieces in a similar manner to the other comparators. The nose pieces are machined with a taper for more accurate measurements.
BASE PIECES SOLD SEPARATELY.
Sinclair, Issue:4A, Page:027