This is the area that really interests me. I was fortunate in being able as part of my job on BR to work a number of manual signal boxes. I took photos both of Allhallows box, which was a nonstandard SR design, lengthened in 1932 with a Stevens frame with 20 levers, and of Stoke Junction box, a Stevens box and Tyers frame with 21 levers.
The signal that started the project of was the four-arm balanced bracket signal at Allhallows. There is a superb picture of it on page 58 of Alan Postlethwaite’s book Odd Corners of the Southern(Sutton Publishing). The signals all operate; those at Allhallows are rail-built, as are the Stoke Junction ones on the branch. Five ground signals (three at Allhallows and two at Stoke Junction) also work. One of the ground signals at Allhallows is a yellow one, giving access to the turntable when horizontal and onto the up line when pulled off. I operate the signals with Tortoise point motors, which give a nice, slow movement. These point motors are actuated from lever frames, one for Allhallows and one for Stoke Junction. The lever frames are fully interlocked with slides and tappets. They come as a kit from Modratec. You download free software called Sigscribefrom their website and work out your own interlocking. What I did was copy the BR plan and interlocking, and then send it to them. After this they send you the kit back, There is a lot of work to do, as the kit is basically all the necessary bits of brass, levers springs etc, But the end result is brilliant, a working lever frame with full interlocking. I then operate the signal motors using switches that the slide bars actuate when pulled.