The M16A2 Buttstock is about 5/8" of the inch for a longer period then the sooner styles. As a result of improved size it made use of a longer buttstock screw plus a spacer. The swivel is mounted closer into the buttplate. This buttstock is currently utilised around the M16A4: https://coltm16a1forsale80111.blogdiloz.com/30534779/colt-m16a1-for-sale-things-to-know-before-you-buy