That looks like a gas can holder to me, with slots for the strap. I dont think they came from the factory with 'em (could be wrong), but I think its era correct. Someone with more knowledge should chime in soon.
Jeff

"Do not walk behind me, for I may not lead. Do not walk ahead of me, for I may not follow. Do not walk beside me either. Just leave me alone, you're starting to freak me out."

1957 GMC 150