Newfoundland Cover - What Is This Registration Back Cancel?

I just got this nice simple cover, with Newfoundland #195.

Registered at St Johns on June 2, 1944 to Los Angeles.

On the back there are three nice cancels at Boston June 8, and then two locations in LA June 11 and June 12.

But does anyone know what is the black ink cancel " HEAD P.M G.P.O"?
Obviously, GPO is general post office...but "Head P.M"?

I've seen this on another cover and think it is somehow associated with Boston entry into the US from Canada

Could it be Head Postmaster?

I believe you'll find that to be a Canadian postal marking (black ink comparable to the front postmark is the giveway). The idea (I think) is to have some sort of a stamp on the back flap of the envelope to insure the contents were not tampered with, since it was sent via Registered Mail. The USPOD did it with the Boston and L.A. backstamps; but the originating post office apparently used the "Head PM/GPO" marking instead. If you search for that marking on the internet, you will find several mentioned -- all originating in Canada.

Many thanks--I think this sheds the light on a small tidbit: Head Postmaster at the GPO from Canada