Does anyone recognize these brackets - What are their purpose?

 Offhand I do not recognize them. It is very hard to understand your photos without any scale reference in them. 

 They do not resemble stock bumper brackets. 

Larry

They are about 9 inches long.  They were in with a few parts that were left from my 85 GT5.

GT5/GT5-S rear bumper brackets the factory used to install the full L type bumper. They were not impact certified so they could not be used on any real DOT certified conversion. In those days some of the conversion shop could cert a 747 to comply, as far as you new!!!!!

Thanks Kirk.  I thought they were bumper brackets just wasn't sure.  At some point before I owned the GT5, the rear bumper was removed.  That would explain the extra brackets with the other parts.

You are welcome, glad to confirm your instincts'.