I can see why you thought they might be quarrying related, but equally why you might think they're not. Especially with that two sided one. It would seem feasible that you might put incisions on two sides if it were bigger, but the time and effort on a smallish stone seems a bit like overkill.

With this: http://www.megalithomania.com/show/image/1762 surely anyone wanting stone would have taken the smaller bits first? Unless they were desperate for a couple of very thin gateposts.

I guess it just adds to the enigma of cup marks.