If its simular to the earlyer key switches that control the heaterplugs and stuff (without relays) then it could be like this (a multimeter can prove it)
5 conections, 4 different contacts. (numbered in tiny rased print next to the metal contacts)
1 massive blade and 1 small blade on same contact - Battery feed (from selonoid, conect to massive pin, the little one is probubly spare (at least on my motor)))
1 normal size blade (6.3mm?) - aux electrics (running positon)
1 massive blade (8.3mm?) - heaterplugs
1 small blade - starter trigger.
If testing the switch with a multimeter it should have 4 positions,
OFF (no conections)
RUN (batt and aux electrics conected)
HEAT (batt and heaterplugs ONLY conected, spring loaded back to run position)
START (batt heat and starter conected, strong springloaded to back run)
Note when heating or starting the aux electrics are not powered so can't be used to operate a modern diesels electric stop selonoid.
Thats how they worked in the 60's at any rate. I don't think they changed it so it might just work.