The M1 Garand rifle originally utilized a screw-on "Gas Trap" system (Circa 1936 through 1941). Due to a number of problems exhibited in using the Gas Trap design, a redesign implemented in 1941 was call the spline-type or "Gas Port". Here's a set of pictures that may prove useful.
More information is included in an article I co-authored in the GCA Journal entitled: "How Many Gas Traps Were Made?" See Volume 24, Issue 4, Fall 2010. If you are not a member of the Garand Collectors Association (GCA), go to their website and join now! Great articles, great pictures, great people! Go to: www.thegca.org.