I removed the oil pump, took it apart and cleaned it up. Here are a few of the differences between it and the later Melling M54DS 60 psi replacement pump.
To begin with it has helical pump gears in it, something I really didn't expect to see.
Melling straight-cut gears on the left, 1959 OEM gears on the right.
The pickup tube on the Mellings pump is .750" O.D. with the 1959 pickup tube measuring only .575".
Here's a look at the pump castings themselves, note the different angles of the relief valves.
The pump cavities look very much the same on both, no real differences that I could detect.
