Signals usually show on power poles in the order that they appear in the signal selection screen. I noticed however that the lamp signal is appearing at the end of the list on the power pole despite being before the end of the list in the signal selection screen.
Refer to the following pic, should explain what I mean.

Edit - I have found a whole bunch that are out of order. So far these appear to be out:
Pump
Train
Fluid Wagon
Lamp
Speaker
Steam Turbine
Nuclear Reactor
Heat Exchanger
Heat Pipe
Centrifuge
Uranium Ore
ALL new barrel products
Uranium 235
Uranium 238
Used Uranium Fuel Cell
Battery -- Appears to be swapped with explosive
Explosive -- Appears to be swapped with battery
Uranium Fuel Cell
Military science pack
Production science pack
High tech science pack
Space science pack
Flamethrower
Uranium rounds
Uranium cannon shell
Explosive uranium cannon shell
Atomic bomb
Flamethrower ammo
Personal roboport MK2
Flamethrower turret
Steam signal
That's all I could find.