the fairbairn sykes knives certainly originated in WW2 but are still in production now , that one is a third pattern made since 1942 if the mark is Shefield ( single F ) it's a wrong un! but i suspect you've just missed an F out, however even if it's Sheffield i would be thinking it's post war and as such not really that valuble proof marks such as the WD broad arrow, B, B2, 21, 4 would indicate an original ( but i'm sure there are others) cheers chris