I think some, if not all of those aluminum tabernacles like Jamie214 describes had a little ridge that the mast seated on after stepping that helped hold it in place in the tabernacle. Actually, friction and the downward forces on the mast will do a pretty good job of "locking" the mast butt in the aluminum casting. Some of those setups had an "L" shaped slot, where the pin would either slide down into the tabernacle as the mast was stepped or would rise up into the slot as the mast was stepped, depending on which way the slot was layed out in the design.
Actually, even with only a horizontal slot, the mast should stay in place due to the way the forces work on it. Most of the forces are tending to push the mast butt down and forward, which will push the pivot pin tighter into the slot.