Is there any difference in gear/body size between the M55 and M55HV pump, or is it just a stronger relief spring?

I have been all over the Melling web site and all I have found is the difference in the springs.

I would "think" the HV pump would have a larger pump cavity so as to provide the additional 25% oil volume they advertise.

The body and gears of the hv pump are 1/4 inch longer than the non hv pump. That’s is what creates the extra volume.