The pen you described seems to go with the round paperweight pen-holders WF made for Parker - apparently a special edition of the 51.
I don't know if that edition was also used for the ashtray pen-holder, as I've only found one photograph of a complete one, and that had a common-or-garden 51 pen in it... next to a paperweight one, also with an ordinary 51 in it. So both of the pens could be replacements (I'll probably have to settle for an ordinary one too, eventually).
Also, the info I've gleaned from WF.com would suggest that they all were supplied with ballpoint pens originally.