Blue =10 Volts.

Lamps can be used, though depending on the number of lamps lit at the time, they may dim slightly.

I looked at a old power supply from a 551(may not be the same as yours) and it has two 24VDC supplies, one marked Signal and one marked Talk. Talk is filtered and Signal is unfiltered.

Sam

"Where are we going and why are we in this hand basket?"