The gun was made at the Baldwin Locomotive Works in Eddystone Pennsylvania.
Eddystone itself was an odd offshoot of Remington (pretty much the pass through sub set up in Delaware) Baldwin put the plant, staffed and setup the machinery together and made the gun (also the British Patten 14 3093 caliber that preceded it)
The punch mark is an oddity, will dig up what I can fine on it. No one really knows, it shows up on a few and not on vast majority. One report is that he saw 3. Random hardness test?
Remington and Winchester also made the P14 as well as the 1917 with some miner difference in fit and finish of the metal and stocks but parts interchange.
If the barrel is an E, it should have the date on it as well top of barrel behind the sight. If not a WWII replacement or a civilian redo.
More pictures and note if there is an E, R or W on the front of the stock on the flat below the sight.
Receiver is a mid run so March/April 2018 - no exact date. they made over a million of them by 1919 with some overun