The RSC 1917, aka FSA1917, has the distinction of being the only true semiautomatic service rifle to see significant frontline infantry use during World War One. It was introduced in 1917 as a long rifle, and about 75,000 were made in that configuration. An improved carbine model was developed in in 1918 right at the end of the war, with only a few thousand of those made. However, what we are looking at today are a pair of 1917 rifles which show a couple differences.
One of these is a standard RSC 1917 as originally produced, and the other has been updated to a 1918 standard in two ways: the bolt handle/disassembly and the bolt holdopen mechanism. I do not know if these changes were actually implemented during the war, or in the years afterward, but they make the rifles substantially easier to field strip.
If you know of details relevant to these changes, please let me know in the comments!