Check the inside of the hoop in the top 'corners'. That was the point that showed the stress of too much 'breathing' on the machine or impact with the court. Scores and scores of the R-22 model were replaced under warranty and I'm sure hundreds more failed over time. The corner angle was eased a bit, resulting in a longer lasting frame.
It was a terrific racket in its day. We sold zillions with the free Yonex towel to commemorate Martina's run ca 1984.