Allows for exact examination of the sear and hammer engagement surfaces. Each base also includes a hole for easy disassembly.
Some Colt AR type rifles have been manufactured with hammer and trigger pins and holes which are approximately .176" in diameter. These are referred to as "large pin" Colts. This was done to eliminate the chances of someone putting M16 fire control parts into an AR receiver. This is not found in all Colt models, just some.
All other AR type rifle manufacturers have stayed with the .156" diameter hammer and trigger pins and holes, or "small pin" which is the same spec used in all military M16 rifles.
Measure your pins or the holes in your lower receiver to determine if you have a "small" or "large" hole Colt AR