Anybody have an idea what this sign was used for?
Easy-- advertising.--If its real the back really makes one wonder. Don't know a thing about signal but companies had signs for tanker trucks,and some sales reps light duty rides looked like poster children when they came calling. I don't think it would be early on as the corners are rounded? The veterans will have to way in on this notion.
If you really want to beat your head against the wall you could track the ownership of home you might get lucky and find someone worked for signal.