and, a "point of order" regarding the use of the term "double proof" - and this gets pretty sticky. using steindler's translation of wirnsberger's text on proof marks....from some point (could be 14 july, 1860; could be 22 april, 1868; could be 1869) st. etienne offered a standard "single proof" (consisting of a single st etienne mark), or a "re-enforced proof" (shown by two marks). at no point in his text is the term "re-enforced proof" presented as a double proof, there are three different places where he states "the double imprint of these marks signified re-enforced proof".
then in 1885, the parliament approved new rules, which were signed into effect by the president - and this is the event that (inadvertently) canceled obligatory proof in france. and until france joined the rest of the proof countries (at the international proof conference in 1914), all proof testing in france was purely voluntary. steindler states that the arms makers never ceased to proof, and that "the french arms industry gained rather than lost ground and prestige". during this period the marks themselves remained (largely) unchanged.
in november 1895, the paris proofhouse was established, and accepted the st. etienne rules for semi-smokeless proofs. as of 30 july 1897, double marks still indicate re-enforced proof, and at some point in 1901, a more powerful proof was introduced in st. etienne (and quickly adopted in paris) - and this where it gets sticky - THREE marks indicated double proof, and FOUR marks indicated triple proof.
"On the 18th December 1923, new proof rules were introduced. Retained was the standard and re-enforced proof which was not identical to either the double or triple proof mentioned above, but the proof marks remained the same. Consequently, it has become almost impossible to attempt to 'date' a gun by means of the proof marks unless the year of proof is known. While high or excessive pressures may well show up a fault, either in material or design of a gun, the exploitation of high pressures as a means of advertising became wide spread in France."
then on 4 june, 1926, to conform with international standards, french proof marks became typical (and much more easily understood)....one mark is standard (14,223 psi), two marks are double (16.356 psi), and three marks are triple proofed (18,490 psi). if you reference the chart that docdrew provides you will see that it pertains to this post 1923 era....and that fact leaves a large number of guns marked with powder T (starting around 1900) and coming forward until 1923-4, that are subject to much conjecture if they are marked with either two or three stamps. based on my understanding, a single stamp will always mean the same thing - and four stamps will always mean a triple proof (sometime between 1901 and 1924). but, two or three stamps are easily misinterpreted....this should give everyone a chance to scratch their head!
best regards,
tom