Possibly a transitional rifle Mike? But as my friend hwvixen points out, it should be in .22 cal so could be a bitsa.
According to Mr Knibbs there were 35,397 "S" prefix Standards made up to the end of 1935 and on 1st Jan 1936 the improved "T" model was launched. I'm fairly sure the numbering started all over again because I have a "T" Standard in the 1100s. But is it possible that yours was one of the very first "T"s tagged onto the end of the "S" model numbering?