They look like the rear legs to me. The two smaller holes bolt the bracket to the seat the single larger hole bolts to the floor. The hole that is in the middle with the bolt in the picture is for the seat belt. I have a 71 blazer that uses four brackets and a 72 blazer that only uses two. Take a look at this link it is a third row seat out of a suburban I am pretty sure it is the same as a blazer.
http://67-72chevytrucks.com/vboard/s...d.php?t=539764
Hope this helps.
Paul