cmc64 has it straight...except that they didn't have either the vocal or piano cluster at the 3/23/74 "Sound Test", and both instead used individual low-driver cabinets arranged along a curved path. The hi-driver clusters appear to be completed for both vocals and piano at this time. The vocal cluster debuted at Reno on 5/12/74, the first road gig for the WOS. There seem to be almost NO pics available from June tour (!) but the piano cluster finally shows up "..."
<<<edited 1-27-14: "...in Jim Anderson's shots from July." I was incorrect on this: Anderson's shots from Dillon Stadium 7/31/74 show individual cabinets for the piano cluster, not the single curved cabinet. Richard Pechner states on his site that Bear's custom piano array was only used at one show, Hollywood 7/21/74 (which would have predated Anderson anyway). >>>
The February 22-24 1974 Winterland was set up very similar to 3/23/74.
Count all the shows from 11/30/73 to 10/20/74 and you've got your number... but for my money, I would extend it back to the aforementioned 2/9/73, since it's essentially the same system, including the out-of-phase mic pairs.