[Snowtractor]

Speculating,
It seems to be a heavy bracket with different notches for tension or settings or lock position. Could a cable have run from it to the back or outside to unlock the door or orient a fixture. Is there a hole which would allow it to operate a sign outside like a breakdown sign or school bus sign? This is the one from Sask that was painted yellow? Snap a few more pics if you can or unbolt it and snap some pics.
Sean