That rear sight was driving me nuts because it was familiar, but was missing pieces. So here we go, from a Savage 1940 parts catalog...
http://forums.gunboards.com/showthre...oject-gun-quot
Look at the Savage ad in post #45, the right-hand page, the 5th one down, the Savage No. 19 Folding Leaf Sight.
"Standard on Models 3-S, 4-S, 5-S rifles"