Hi Paul,
I cannot find any information on your switch on the web. Doing a search on the Santon switchgear website sr-137, sr 137 or sr137 are not found. For anyone to help you a picture of the switch showing all the connections would help. Also if you use your multimeter to work out which connections show continuity to which other connections with the switch in each of its three positions and post this information then I am sure there a number of members of this forum that can help.
Les.