TheAustin crankcas is very distinguishable from the Bantams because it has a "batwing" web between the front and real motormounts (the two holes at either end of the bottom of the engine.Always check the motor mount ears for cracking.
The Bantam two main has no "batwing" reinforcement between the mounts but had additional reinforcements in front of each of the mounting holes. Also note the dianond shaped freezehole(?) in the casting and the additional obtrusion above it.
This is a badly damaged 3 main but note the mountings are gone altogether and just raised rectangles take their place. The three main was mounted tricycle fashion, with one monunt in front and two back on the transmission.