M1903 NM MADE IN 1918

[Cosine26]

It the receiver was actually made in 1928, it should be of nickel steel. If of SA manufacture the letters "NS" will be stamped on the flat behind the recoil lug on the bottom of the receiver. If the receiver was made at Rock Island, it would be before 1928 and, if of NS would be stamped "NS" on the front of the receiver ; however, it is necessary to remove the barrel to see it. It also looks to me (hard to see) that the stock lacks the forward stock bolt which would make the stock fairly early - well before 1928. The butt plate is hard to see but it may have the fine checkering present on pre-WWI rifles.
I do not know where they came from, but many of the old SHT receivers were available on the commercial market before WWII and were used by some small companies to make cheap sporters. The markings were removed to hide the fat that they were of the SHT. R.F. Sedgley Co. was noted for this and offered many but none used the "S" stock. Look through the preWWI American Rifleman magazines and you will see them advertised.
FWIW