The base cover is the piece you sit on. This design has simulated stitch lines that pressed into the vinyl with a high frequency welding machine. This was the original Factory design. From 1960 until the mid 1970's, all non reclining front seat frames were the same. Fitting the seat covers, seat diaphrams and foam/fibre paddings will be same throughout this period. Basically, the diaphrams come with hooks to clip onto the frame, the squab padding kit and base foam pad are glued to the frame. The seat covers once fitted are held to the frame with metal clips. We would suggest that you strip one seat first to see exactly how it is fitted, then assemble the diaphrams and foams etc and then fit the covers in a "dry" run before final fitment.