I've since lost it but I had a photo of York firing an M1 with a US Army sergeant looking on. The caption is York likes the new rifle.
Not accurate, he was well known for his marksmanship and was tasked by his Captain to train others. I think it is plausible that the marksmanship expert of the unit could carry whatever he liked and the 1903 was the official rifle. One account mentions he had six rounds in his rifle and therefore it must have been an M1917. The M1903 holds five in the mag and one in the spout makes six. That's exactly how I'd have carried a rifle in combat back then... well now too! Some will also comment on the statue of York in Nashville, the construction under the auspices of his family, that shows him firing a 1903. 5mad, just playing a little devil's advocate, I'm not going to get into a peeing contest ;-) I suspect York could have pulled off his achievement with any rifle in use at that time. The 1911 is what saved his bacon after all.
Prior to October 18, 1918 York was "Corporal, Infantry, 1 each, nondescript and expendable".
Phillip McGregor (OFC)
"I am neither a fire arms nor a ballistics expert, but I was a combat infantry officer in the Great War, and I absolutely know that the bullet from an infantry rifle has to be able to shoot through things." General Douglas MacArthur