I don't even have a SWAG as to what that card is.
I'm a little dubious
about it belonging in a PDP-8/a, but I suppose anything is possible.
Well, IIRC, there was some kind of connector/clamp just above the 'F'
edge connector fingers. If the board was put in the normal 8/a box,
there'd be access to that connector through that hole where you connect
the console cable, etc.
So it might well be for an 8/a
-tony