Those '47.5-'55.5 dual tail/brake lamps for panel/suburban/canopy bodies were a factory option (#249) and are hard to find. These were required equipment in some states at least as early as 1950 (in WA, at least).
In 1954, there was an accessory turn-signal set that came with the housing-cup/switch/knob for mounting under the steering wheel and a wiring harness for tail/stop/signal lamps (also came with wiring and dual-filament bulb holders for the front parking lamps).
Chev's of the 40's used to sell the body-mounting gasket/seal for the dual tail/brake lamp, and also the chrome rings for the lenses and the gasket/seal for the chrome lens ring. As stated above, there does not seem to be a reproduction for the housing but the units show up every so often on eBay.
I was lucky and found a complete NOS 1954 Suburban set in Hemmings in 1997 (complete with fold-out 12-page installation drawings and instructions).
Here are photos from the side and rear, and of the switch-arm (1954).
Tim
1954 Suburban rear signal lamp - from the rear 1954 Suburban rear signal lamp - from the side 1954 Suburban rear signal switch under the steering wheel.
