Bucket Seat help

I can rebuild trannys and engines but I can't remember how to attach my bucket seats ('68 cuda) to their track. I have restored the tracks and the front two mountings are 5/16" studs w/ nuts. The rear two have a square hole- what goes in there to attach the rear of the bucket seat? I must have lost something that goes in there. Thanks for the help.View attachment 1715813022View attachment 1715813023

should b same as the front ones --------