I just went upstairs and got my 1922 Colt Army Special, which is essentially the same gun (bar some cosmetic differences). It also has an H stamped inside the frame, though I'm fairly sure it never belonged to HPD.
A good start might be to work out when it was born. On mine the manufacturer's S/N can be found inside the frame, when you flip out the cylinder. You might want to check there as well as on the butt. Once you have it, you can look up the year of manufacture here:
http://www.proofhouse.com/colt/index.html. Interestingly, the Official Police #s continue on from the Army Special #s (which themselves follow on from the New Army & Navy #s).
Where I'm going with this is, I think a solitary "H" can be explained as just another mark that I'm too lazy to look up. But, the HPD No. XXX might well be a secondary serial #, put there by or for HPD and, if you can work out when it was made, that will give you a definite era within which to research HPD practices regarding marking issue weapons.
