There were several water pumps for the XK120 and Mark VII up to the end of '54, and it looks like you have one of the later ones, although not as late as my 1960s replacement, in that you have the hub with two ball bearings and driving dog, where I have the bearing shaft assembly as was used in XK140 and later pumps.
These are some versions; there may be more:
- without top port, 5 bolt hub
- with top port, 5 bolt hub
- without top port, 4 bolt hub
- with top port, 4 bolt hub
- repro with bearing shaft and 5 bolt hub (top port unknown)
- repro without top port, with bearing shaft and 4 bolt hub (mine)
You can run your bypass into the top port with that nipple you have.
If you have a heater, you will need a banjo, copper gaskets and banjo bolt.
Or you can get two banjos, more gaskets and a longer banjo bolt and run bypass and heater from the side port.
Or you can run without a heater.