These fit on my Colt 6920 upper and lower. I have one on my Colt A2 carbine upper and a modern lower with minimal contact with the hardware I use to assemble them. They can be ground, but I don't grind them. I can assemble them and give them a patina coating black coating for 1980s look or leave them in original "gray" and oil them for a old school patina.
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