Welcome to the forum.
Per the Red Eagle News Exchange, which our member Chet 15 puts out,
your Single Six is a 1957 Type 4 Flatgate (RSS5). Has a large round (with concave face) ejector rod button and a flat loading gate. S/N range 50598 to 61960 Approx. 11,000 made.
Hope this helps ya a wee bit.