In 1966 Colt sent about 3,000 10 inch barrel XM177E1's to Vietnam. By January of 1967 the Army had decided to increase the barrel length to 11.5 inches (XM177E2) and use the same 4.25 long "Moderator" flash/blast suppressor- but adding this "Grenade Ring" between the barrel and the Moderator. The point was to allow attachment of the under barrel Grenade Launcher, which was not possible on the E1.
This is primarily an XM177E2 / GAU-5 A/A only accessory, but was available on all 11.5" Colt Commando models thereafter
- Here is the Crush Washer used to time your Moderator.. just in case you need another one (Here)
Note: Picture shows: Lock Washer, Crush Washer and Grenade Ring. Only the Grenade Ring is included in this Order. Others shown are for comparison only.