Since bottom activated cut levers 'replaced' the top activated ... what was the advantage of them over top activated? Was the pin more likely to drop correctly with bottom operated or something like that?
As Bruce said, both the Type D and later Type E could be had either way, but bottom operated came to be preferred. Top operated had an opening that allowed water, ice, and debris into the lock mechanism, which could then jam. In addition, the clever pin lifter linkage of the bottom operated style provided a mechanical advantage that made them smoother and easier to operate. We had both types at the railway museum where I was active years ago, so I've worked with both types.
