Good for you Ross for tackling the M113 mount in 1/25. The Academy mount is simplified and pretty basic when compared in detail to the real thing. The way you described the mount is correct. There are also locking mechanisms on the tubular part that the pintle drops down into. The base of the legs are multi-angled and not square like the Academy M113 mount.
There is no change in the leg base mounts when it was mounted on the Centurion commander's cupola on the Mk.11LR.
For an M113A2 the most visible difference from an M113A1 are the external fuel tanks. There is no internal tank with the M113A2. When the external tanks were added to the M113A2 the standard fuel filler cap and armoured cover on the left rear upper hull were removed and blanked over. You will also see the later style front signal lights on the M113A2...they are the bigger version like the rear tail lights on the M113A2.