S-G refers to Sergeant-Greenleaf, if I have spelled it correctly. They make switch locks. I believe it is their brand which uses the keys you describe - the 107, 102, and similar numbers.
I once had a salesman tell me they were totally resistant to freezing up, just could not happen. That was when they were first coming out. In the years since I have found them every bit a prone to freezing as the older style. The older ones could often be opened after giving them a sharp blow with a spike or something heavy - to break the ice, not the lock itself. The S-G don't seem to respond quite as well to that treatment.
Years ago one shortline started up and bought a whole box of regular Master padlocks keyed alike and put them on the switches, figuring it was a name brand, less attractive to railthief collectors, etc. Winter arrived and the crews began to use fusees to melt the ice. The fusee also melted the brass internal parts, causing the whole cylinder to fall out and render the lock useless. An order to Adlake for traditional switch locks soon followed.