The proof marks Left to Right, NOT specific to Cordite, semi-smokeless, or other “Nitro” powders:
- London Provisional
- London View
- London Definative

Express Rifle Proof mark
“.450 EX” (Express Rifle, once again only Black Powder Proof unless stamped otherwise)

“GI37P” may be a Holland & Holland or barrel maker number?

“N P” is not a recognised Nitro Proof mark in any of my sources for any Brit or European proof. Tellingly the letters are separately struck and don’t conform to British proof law standard which would be “Arm with Sword/NP” if London Proof and “Crown/NP” if Birmingham Proof......all on one stamp.

No “re-proof” marking as would be expected with re-proof to Nitro by a recognised proof house.

The other anomaly with claimed Nitro Proof is that there is no maximum powder/case/bullet weight stamped on the barrel flats as required for British Express Rifle nitro proof.

From visible evidence this rifle surely does NOT look to be “Nitro Proof” as governed by competent proof authority.

This is your deal; however I would suggest avoiding trouble with hearsay claims of proof & contacting Holland & Holland to get WRITTEN confirmation of details of this rifle. Look for actual build/sale date and any information as to known re-proof status.
I would think that at least here in the States, with liability concerns, that having successfully shot it as it came from a relative is different than claiming Nitro Proof for a sale then the eventual buyer having a serious problem.

Just my 2¢, would love to see what Curl, Cal Pappas, Steve Bertram, or other knowledgeable DR enthusiasts might have to say.
- Mike