I've just discovered that Rob and Abby Mouat's Cornell Publications offers a free listing of proof marks at http://www.cornellpubs.com/Catalog/proof-marks-intl.pdf.
Not only do the Mouats offer something like 5,000 reprints of gun catalog from makers and merchandizers, but their newsletter is free and filled with interesting material, some relevant to firearms and some not: the current one has a video of a Toyota SUV being driven on two wheels while five or six Saudis take off and then replace the two other tires--incredible! I'm hoping my grandson doesn't see it! And the Rants and Raves every month are always worth a chuckle.
Check them out: they provide a wonderful service at very reasonable cost.
Added 8/11/2016: Another useful reference is Gerhard Wirnsberger's Standard Directdory of Proof Marks, With WWII Germand Ordnance Codes. Translated by R. A. Steindler, it's distributed by Blacksmith Publishers Corporation.