Some Colts have "small" pivot pins and some have "large". Adapters are made for either type of mixing of the two different sizes, such as "small-hole" lower to "large-hole" upper or vice versa. Obviously if both lower and upper have the same size pivot, no adapter is needed. Luckily, my pre-ban Colt lower was the standard "small" pivot and it mated up beautifully with my Bushmaster upper.