I have the same pump. It is a Stuart Turner No. 10 Centrifugal Pump described on page 27 of the 1968 Stuart Models catalog. The451994 number is a patent number.
This is not a model pump, rather it is a small pulley pump meant for pumping small volumes like a shower sump pump or a fountain pump.
I would suggest having a machinist cut off the broken threaded pulley shaft, drill and tap a hole in the end and screw in a new threaded shaft. The pulley shaft and nut are a left hand thread to prevent the nut from coming loose.