AW-SIM, here are a couple pics, one with the safety in place and one with it removed so you can see the machined slot and notches. This one belongs to a member of this board, but is not mine. Hope he doesn't mind my posting this, but he will remain anonymous. I do have a couple of examples, but with no safety. This is the only example with a safety I have seen, but I am sure a couple more have survived. The soldiers kept throwing them away, so eventually procedures were changed and they were no longer fitted. These original 1917 back plates are certainly are rare, with or without the safety itself.
Dan, since I just saw you slipped in there, I believe the aluminum grip first appeared with the 1919A2 cavalry gun, and so was indeed the first version used on the A4.