Paul, do you mean the Bee hucard copiers? I don't think they're used for PC
Engine hucards as someone tried one and it didn't fit into a PC Engine. Maybe
what you saw was something for some other console that used similar shaped
media like the old Sega machines.
http://members.hknet.com/~odie1974/Bee-card.jpg
http://members.hknet.com/~odie1974/Beecardm.jpg
http://members.hknet.com/~odie1974/Beecardp.jpg
I managed to nick the yahoo auction pictures just before George won it ;)