It's my understanding that firearms have to be commercially proofed before that can be sold commercially.
Military guns are occasionally found that do not have commercial proofs because the owner bought it while in the military and never sold it through a commercial shop. (I have a .455 TL that was purchased/issued (officers had to purchase their own sidearms from the Army/Navy store)by a Captain in the Medical Corps. When discharged he went back to Australia and took the gun with him, so no Comm/ proofs.

I might guess that your .44 found its way to Britain in private hands (or a direct order to S&W) so it wasn't proofed upon arrival. It stayed in private hands until 1952, when someone decided to sell/export it and had it proofed.

This gun really needs a letter!!

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Dean
SWCA #680 SWHF #446