SN looks like 686841, dating its probable shipment to be around March-May 1940. And at that time, the pre-war commercial Carbonia bright blued finish was still used. As were the silver medallion checkered wood grips. A letter will not likely tell you much information of value beyond the exact shipping date. It will probably show it was shipped to the British Purchasing Commission, New York City, and how many guns were in the same shipment. Due to existing U. S. neutrality laws at that time, the British had to pay for U. S.-manufactured weapons and war materiel in cash and were responsible for transporting it to England or wherever ("Cash and Carry"). Late in 1941, the Lend-Lease Act of 1941 took effect, which essentially allowed the U. S. to lease (really meaning "give") American war materiel to various allied nations. And did we ever.
"Hopefully I can obtain a new keeper pin when I order the butt swivel. "
It should be simple enough to find a piece of wire or a small nail, paper clip, etc., of the correct diameter which will work fine. You probably won't need to buy a pin if one is not included with the swivel. There were some minor differences in swivels depending upon when manufactured, but that's of no real concern.