I found this in the cab of my truck. Does it bolt to the license plate?

 

My Oregon 1952 plate has one somewhat like the one you have in the pic.

 

Yep- That's a plate tab. Not sure exactly which state, but good used tabs are worth a few bucks to guys needin 'em. There would be a slot that the narrow part would go into on the plate and then one of the plate's mounting holes line up with the hole in the tab. Depending on what year and condition Washington tabs go for anywhere between $5-$30.

 

1965 Minnesota farm truck license plate tab pair with matching serial numbers | eBay

 

I'd say you've got a really cool little wall hanger for your garage. Or living room. ;-)

 

Put it on the key chain!