The "slot" drive oil pump has been the factory standard for many years. The evolution of the pump in production was started with the "pin" drive that is most common in early small bore motors.
These pumps have a fork on the drive side to engage the "pin" in the end of the camshaft.
Many 1275cc engines were then given "star" drive pumps. The pump has a splined shaft that engages a "star" drive that fits into the camshaft.
The latest slot drive pump has a screwdriver end that engages a "slot" on the end of the camshaft. Almost every new or reground camshaft now available from the UK is only available in "slot" drive configuration.
Oil pumps in the Mini/Sprite/Morris motors should be the most replaced item inside the motor. NEVER open the motor without replacing the pump.
In these motors the pump is asked to suck the oil directly from the sump and send it to the oil filter. Thus the pump only sees UNFILTERED oil and does wear out quickly relative to the rest of the motor.
As a rule of thumb you should have 10lbs of oil pressure for every 1,000 rpm's.
Related parts:
For oil pump gasket, see 12G703B