My reason for the question is functionality. I bought a Colt HBAR (A2 I presume). Then I replaced the furniture with with a old Colt wood set up. I put everything together but the bottom screw wouldn't engage the sling swivel (too short). Bought the longer one and thought all was well. Finally took the the rifle out to sight in the scope. When the range master called a routine cease fire, I discovered that the bolt catch would not engage, to hold the breach open. I managed the rest of my outing with a spent shell holding the breach open when necessary. Then came my cry for help from AR15.com members. The most simple trouble shooting method was to partially unscrew the top screw in the buttcap. Viola! Problem solved. My problem now is, I dont know if I relaced an A2 set up with an A1 or visa versa. From the look I got from the guy at the gun show when I bought the lower screw replacement, I want to be able ask for a A1/A2 upper buttstock buttplate retaining screw. Asking for the lower, stock cap, thinggy screw, longer than this one, just didn't go over well last time.