That's a good question. You'd have to be an old fart like me to actually remmember... I'd have to look it up!!! I bet it was a Fender (Vibroverb) though...
Guild Thunderbird: 2 7591s, ss rect for about 30 watts. A (Celestion 75?) 12" and a CTS 8" (the reverb channel speaker). The reverb tank doesn't exist...oh wait, it's in the chassis! Guild liked to experiment a lot.
The band probably had a sweetheart deal with Guild since the Butterfield band's organist, Mark Naftalin, played an embarrassingly cheesy Guild organ through his own Guild amp.