+2 on "probably nothing" or just plain "nothing" but if you think it's am RPM - "repunched mint mark" then check here and compare the examples to yours ==> http://www.varietyvista.com/09b WQ Vol 2/RPMs 1958-D.htm
I was looking for info about the mint mark on a 1958-D quarter I have and I found this thread from last year so I am bumping it up and adding my image. I think the mint mark on my coin looks a lot like that on the OP's. What could have caused this? Thank you!