Asked the curator at the Birmingham munitions museum about the proof marks on my 35 Rem, this was his response.
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Hi Joe,
Up to 1954 all foreign weapons imported into England and proven here were all stamped “ NOT ENGLISH MAKE “ one can only assume that it was imported from America
Prior to 1954 as the proof marks on your rifle were in use between the years 1925-1954.
I am sorry that I have no more information for you regarding lend lease, however if it had been I would have expected to see some form of military marking somewhere on the weapon.
