I bet it is an M (notorious agraffe poppers in that era).  File the
tip of a screw driver into a \/ or /\ , where the end has two points
(as a fish tail).

Lightly tap this into the post and turn.  Do not expand the brass,
just catch it.  WD-40 (sparingly) or other lube oil may be needed,
as well as a little heat.

Another way which has worked for some is to use a reverse twist drill
bit.  As it drills in, it has a tendency to unscrew the post too.
In the spirit of overkill I also run a tap through the threads.

If a slot had been cut in the bottom of the post prior to installation,
then all that would be needed is a thin screw driver to run it out.
Hind sight, no help now.

Good luck

Jon Page,  Harwich Port,  Cape Cod,  Mass.

 [ M?  Whozat?  -- Robbie