I have a set of Model A mud flaps that uses a spring and two sliding plunger like bolts to attach to the inside of the bottom of the fenders. The mud flap has a flap sown into the top of the flap that houses the spring and the two plungers.
I'm sorry I can't show a picture of the flap at this time as they are in my shop in NC and I work out of FL. I have had the flaps for about 45 years. I have no idea who made them but they fit the Model A fender perfectly.