THe bible is in the truck, so I'll give an opinion.
THe photocell is more than likely not being used according to mfg instructions, if it's rated 120 volts.
Breaking one leg is not a good practice, and leaves an energized conductor somewhere btwn the PC and the sign. A sign tech may think the sign is de-energized. This is similar to using a single pole t'stst (line volt) to control a 240 volt bb heater; or a s/p switch to "shut-off" a 240 volt circuit.
"It works, don't it????"
(Famous last words.