by Mike Hodgson » Tue Oct 31, 2017 12:48 pm
If you are modelling SR or BR(SR) signals I suggest you get hold of a copy of "A Pictorial record of Southern Signals" by G Pryer. Probably out of print but available from time to time on Amazon or ebay. The backs of signals have very rarely been photographed as such, but you can often see them as background where they appear incidentally to photos of something else such as a loco.
It is very hard to generalise. Quite often Southern Railway or pre-grouping signals survived into BR ownership and practice varied.
Wooden signal arms needed repainting from time to time and replacement due to rot; metal/enamelled arms less so, but even those could need replacement due to fading or damage.
Rules 44, 45 and 46 in the 1950 BR rule book relate to Call-on, Warning & Shunt-ahead signals respectively, and some but by no means all signals bore the appropriate letter. Some signals fulfilled more than one of these functions, in which case there might (but also might not) be a C, W or S indication associated with an otherwise plain miniature arm with our without vertical stripe. Subsidiaries showed a small red or no light in the on position, a green light or a white light when off.
Some (Central Section) calling on arms had black ring and also had white vertical stripe on the face.
On the Eastern & Western Sections the circle was replaced by a red diamond either with or without white vertical stripe
I don't know whether the black horizontal bar on the rear was peculiar to WR, but I very much doubt it (and probably not universal on the Western either). No doubt somebody will be able to cite specific examples. As for Westernisms, the Regional boundaries were not 100% match to the original Big 4, and some places moved from one region to another at various dates, so the BR(SR) certainly inherited some ex-GWR signalling.
ER and WR also inherited long-lived pre-grouping equipment and were affected by changes regional boundaries (not least the abolition of the NER) so they also did not have 100% standardisation.
I think you will get a more helpful answer from other if you are able to be more specific than just the region and an implied date range - though I recognise this many not be practical if your prototype is hypothetical.