BobM2:
As I recall, the third or standby attitude indicator, powered from the battery buss, was added after UAL 266 & was mandated by FAA on all jet transports, not just 727s
That could very well be the case. But, on the date of the accident a 727's standby power would have powered the captain's
normal instruments plus (I think) VHF 1 and NAV 1.