Our Rules and Regs books about 3 editions back listed in the front that they were adopted by SVRy in 1914 I think, with the modern revisions listed starting in the 1980's. The actual SVRy rule book from 1914 that I have seen had a lot more pages, but was still vest pocket sized.
From time to time in the yard we relay signals by way of a person on the ground. In some cases such as spotting the coach inside the leanto side of the carshop, there is a person on the ground inside the building relaying to a person near the doorway outside, relaying to the switch engine.
I remember some switching we were doing a couple years ago for the fall photo freight where due to visual obstructions on the ground plus curvature, signals were being relayed on the fireman's side. I was fireman, and simply yelled the signal across the cab to the engineer (Paul) As long as everybodies on the same page it works ok. We seldom use the radios in switcing as noise in the cab can make it hard to hear and I think in some cases their is more potential for misunderstanding. If the person on the ground gives big graphic signals, it is pretty hard to mistake a forward/highball from a backup or a washout. Unfortunately some people give mousy little signals and it is a pain in the butt.