Ok, here's what I found out:

The pump has the following numbers stamped on a smooth surface on the bottom:

V120 25 213 10
0-21 S203

The pump's centre "ring" casting that envelops the rotor (it's what's cracked, and comes in the repair kit) is exactly 3" square.

The tractor itself is a Cockshutt 550.
Model #   45-0787
Serial #   140603-519

Hope this is the info you need! Thanks again for your assistance.

 - Robin.