I don't have my documentation handy, but I believe all TR4A should have the 5/16 exhaust stems and 2 springs. The third spring was used only in combination with the heavier 3/8 stems. AFAIK the 2-spring setups were left to wander around within the counterbore on the head, the locating pedestal was used only with the 3 spring setup (and only fit snugly on the early guides, which had a larger OD than the later ones).
I haven't seen a lot of wear in that area; is it possible that the springs were replaced with ones that were not ground flat on the bottom? At any rate, I don't know of a spacer to locate them. Any speed shop will sell you shims that would compensate for the wear (and prevent future wear), but be wary of coil bind especially if your camshaft has more lift than stock.
I also would not even consider re-using valve springs, especially if I didn't know their history. They lead a hard life and original springs are almost certainly sagged by now. They can even break, if the alloy wasn't quite perfect to begin with. And finding a 4A head with 3 springs suggests a certain carelessness, to me.