The show had a number of office run-throughs, evidently-- Tom Poston & Kitty Carlisle took part in some of them-- but I've never even seen **reports** of an actual pilot episode. Since the show started off in the CBS daytime schedule, however, it may be that any pilot that was shot was utilized as part of the initial run of eps that are now presumed lost.
I still marvel at the IGAS episode where G-T cross-promoted the recently-debuted "Password" by having Vivian Vance come out and show everyone how it was played...
-Kevin