I'm no expert either, but you might be able to tell somethings about your stock with a heat treatment test. If it is 4140, it has 0.38 - 0.42 percent carbon, which would mean that it would harden considerablly on heating to cherry-red (non-magnetic) and quenching. A small sample of it might be cut off for the purpose to simplify the heating/quenching. A low carbon steel like 1018 would not harden much at all. As for the 41 part, 4100's are low alloy steels . . . a small bit of Cr and Mo . . . strengthens the steel, but you're not going to be able to identify their presence in the home shop very well. For details on alloy steels, you might try:
I guess I would say that if it responds to the hardening test like a medium carbon steel then you should accept the 4140 marking as your best guess. If it doesn't harden, it might as well be used as a mild steel anyway.
Pi = 355/113 . . . to
<85 parts per billion
Pi = 355/113 . . . to less
than 85 parts per billion!