I'm aware of only three types, but there may be more. The earliest ones were solid lugs with sort of a step for the locking surface. They used a separate pin which rode in a cut out in the lug and was captured by the barrel lug to actuate the lugs when the trigger guard was pulled. The second style (of which I am aware) used the same type of actuating pin but was split longtitudinally and had a more tapered locking surface. The latest style (again, of which I am aware) has the actuating "ears" integral to the lug and does not appear to be split longtitudinally. If anyone has seen other styles, we'd all like to hear about them.
RonF