Sir,
The 2 little brackets with puny sheet metal stampings are to hold the front of your seat frame to the floor. I know that a couple of 1/4" bolts seems like overkill, but that is what was used at the front. The seat slides into a much wider bracket for the rear of the seat, then those 2 small tabs hold the frame down to the floor.
You are indeed missing a strap that connects the lower tubes of the seat frame. These must have been welded on the afternoon shift, beacuse mine fell of also. Or maybe just a fat guy leaned back too far - this also happened to my '03 Silverado
.
Good luck.
Stewart