That's the one mine has. A nice clear "3GM-K" in a rectangle. Wish someone did know!!
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This is really cool information thanks!
I found a footnote in C.F. Ferris book that Colonel John T. Thompson received a letter from a British LTC W. H. Webley Hope dated 25 May 1917 objecting to the new modified "Enfield" being called an Enfield due the large number of changes to the rifles and that the British Inspection Specification for the Pattern 1914 does not use this term at all. Additionally, LTC Hope related an incident in Washington, DC that highly placed officer thought the new rifle was a Lee Enfield. Col. Thompson placed a 1st Endorsement on LTC Hope letter dated June 7, that the the Rifle will be known as U.S. Rifle Model of 1917. The reciever will be U.S. over MODEL 1917, over the name of the Manufacturer and than the serial number. During a Mnaufacturers Meeting on June 25-27, 1917, Thompson added the "OF" in the Model, to read "MODEL OF 1917". Still people still called the Model of 1917 an Enfield even publishing a WWII Field Manual with Enfield in the title. I believe there were still folks in the US Ordnance department wanting to remind folks that the M1917 was "not invented here".
From what I can find out there appears to a definite record of the serial numbers being stamped with the U.S and W. probably up serial# 4663. In the serial number there are notes that between July 12 1917 and August 23, 1917 no new serial were recorded. However there are note to the effect "12 bodies numbered 4025 to 4036 July 20, 1917" and 15 Bodies numbered 4050 to 4065 Aug 9 1917." The notes indicates that these bodies were for experimental purposes. A rifle with 4064 appears to be an experiment with the change from the US and W to the full Winchester name as an experiment. It was another two weeks before serial numbering started back up again probably to make the new roll marking die. The serial numbering start again at 4664 on August 24, 1917 with a note. "No.17. old fashion - Last No. - from 4623 to 4663, inclusive." So, 4663 is probably the last serial number of the U.S. over the W with a number of at least one below probably more as experimentally marked WINCHESTER. But there are always one or two that goes against the grain. The information was taken from C.S. Ferris book.