It clearly shows rifles that are European and subject to an official proof testing are stronger.
IMHO using a brand of rifle that isn't subject to this sort of official testing is akin to driving a make of car that has never been crash tested and under the scrutiny of an official testing and safety org.
ALL firearms sold to the public in the UK MUST have been proofed and marked as such before they can be sold. As you have pointed out no American-made arms APPEAR to have been proof-fired, although I'm told by one manufacturer that a number of guns are randomly selected and proof-fired but not allowed back into the sales chain afterwards.
ALL-US-made firearms are 100% proof tested in England [Birmingham or London Proof Houses] before being offered for sale.
Austria Austrian proof marks were accepted in the UK pre WW II, but only marks post January 1956, are now valid.
Belgium... The specifically recognised marks of the Proof House at Liege only.
Czechoslovakia... Marks of the Czech Proof Authorities have been recognised in Great Britain since 14th October 1963.
France... Proof was optional in France prior to July 1960. Only specific, agreed, marks applied after that date are valid in the UK.
Federal Republic of Germany... new marks from September 1955 are acceptable, but marks prior to that date are not now accepted.
Italy... The current marks are accepted.
Ireland... Irish proof is acceptable after 9th June 1969.
Spain... Spanish proof is accepted.
The products of all other countries, including the United States of America, must be submitted for proof within 28 days of importation.
tac