Pryor's 'Pictorial Record of Southern Signals' which is my normal 'go to' for any questions on SR signalling does not give any dates for the introduction of rail built posts. I would have thought that pre-grouping installations would still have predominated throughout the twenties unless there was significant layout remodeling and as for lower quadrant arms on rail built posts, Im not saying it didn't happen but I think it would have been rare.