I would'nt call Spacelab a "conversion". It was just the add-a-ball version of "Skylab". Both games have the same playfield, notice on "Skylab" you have to spell "Spacelab". From what I remember hearing Williams made alot of "Spacelab" backglasses that ended up in "Skylab" machines.

To answer your question the game really isn't that rare, because they are basically the same game. If I remember correctly "Spacelab" is easily converted to replay, I think they even shipped with a credit wheel. Does yours have a credit wheel?