This set of hardware is used to hold the seat frame to the rails and the seat rails to the floor mounts.

They are steel with a hardness rating of 12.9, the same used on many axle bolts, a lot harder than the stock bolts or anything you will find in most hardware stores. They will not easily strip or round out like the standard bolts and are slightly longer so they are easier to fit. 

928 (85.5-95)
944 (85.5 and up)
911 (85.5 and up)
968 (92-95)