I have Vol. 1 (1983) and Vol. 2 1986 of the SRS "Serial Numbers of Martial Arms", just based on where the information relating to M1ser. numbers came from, I would say no, at least not in my books. Vol. 1 indicates the main sources of information on M1 rifles were the post WW2 records of SA and records of the DCM, which show mostly NM rifles. In Vol. 2 the main source of data is from pre WW2 records of the USMC. My Vol. 1 only lists six rifles in the 400,000 range and they are all records from the 1950s.
I would guess the reason some have indicated they would rather the BritishProof markings be at the rear of the barrel, behind Op-Rod is because those rifles are the only ones you can be sure are, in fact, lend lease. Those marked in the front between rings of gas cylinder may or may not be lend lease. While yours certainly is within the correct range, you just can't tell for sure.
As far as the lock bar sights, they were a upgrade from the short-pinion/flush nut. My guess is that the original sights w/ flush nut on your rifle were not removed by a collector looking to make a few dollars, but were changed back in the 1940's.
Are you aware that you can tell the year your rifle was proofed for export at Birmingham by examining the "date code" in the left angle of the "crossed swords". They started with the letter A for 1950 and progressed (omitting the letter I ), to the letter Z for 1974.
In any case, you have a beautiful rifle and I would think long and hard before spending the money that is being asked for short pinion/flush nut sights and ligit SA/GHS stock. And before I bought a stock I would want the opinion of an expert. Just to many fakes floating around.
As someone else said, it is what it is, and anyone would be happy to own it.
By the way, the rifle in my AVATAR is 422881. From what I can see of your parts, they are the same, except the heat lot on my -2 bolt is RE4A