Way back in 1956, Librascope's LGP-30 had an "R" instruction ("store Return address") that stored (already-incremented)PC+1 into the address portion of the inst

Way back in 1956, Librascope's LGP-30 had an "R" instruction ("store Return address") that stored (already-incremented)PC+1 into the address portion of the instruction at the destination, which was by convention an unconditional branch just in front of the beginning of a subroutine. You'd follow the "R" instruction with a "U" (unconditional branch) to that subroutine. The subroutine would return by branching to the address just in front of it, which conveniently was an unconditional branch back to just after the call location. So, recursion was out of the question unless you used some more advanced calling convention. (And all opcodes in assembly language were a single letter.)