Can anyone explain the purpose of the rubber "flap" at the rear of my old seatrand 70 board?

patents.google.com/patent/US5683280

Stopping cavitation.they were a good board in their day.

Interesting Ken Winner was the inventor and he has invented quite a few windsurfing and kitesurfing innovations and holds a number of patents.

almost purchased a Roberts 28 flapper a couple of years ago for only $200
but the flapper was busted - like the one in the photo
here is some info and i am guessing the flapper was a kind of precursor to the tail cutouts - helping move the fin back on the board

groups.google.com/forum/#!topic/rec.windsurfing/BLonvmaUw-g

Yep AVS = anti-ventilation skirt.