It is made that way because that leather welt is heavy, and on an inside corner if the welt just had straight cuts, it could overlap itself and make a bulge. This way you can sew it to an inside corner or an outside corner and it won't overlap itself. That was sewn on a dedicated machine that sewed the welt and had a cutter to make those round notches. There are all kinds of dedicated machines in a sewing plant that only do one operation. The parts of your Cadillac seat from that long ago would probably have had the edges of the seat parts run through a serger which would have cut off some of the fabric and made an overlock stitch on the outside edges as it sewed the leather to the sew foam underneath.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 01-28-2013 at 04:20 PM.