The first bracket was a box that said "electric seat bracket Riviera" and the person to whom it belonged passed away. So, I have to assume it is what the box said since the person owned a 1981 Buick Riviera (which I already sold a year ago). It is stamped D4982 so that may be its part number. It may be the same for other GM vehicles. It is 8 inches by 2 3/4 inches. $15.
The second bracket was loose and came out of an automotive air conditioning shop. Don't know for what it is. It is made of 1/4 inch thick aluminum or alloy. 10 1/2 inches long, 2 inches wide. Distance between the round holes is 9 3/8 inches. $10
602~697~1877 (OK to call 07:00 AM - midnight. Hablo español.)
(I cancel each ad as the items sell. So, whatever is listed IS still available.)