That's what I was thinking until I saw the photo below. Looks like there is a panel right behind the seat, unless it's open below the shoulders.
I’d speculate that the absent pilot was due to a failed seat structure under the high G forces that followed a control surface failure. Other photos of the same plane and pilot have his helmet nearly scraping the inside of the canopy.
The seat was probably something light and relatively fragile for racing.
The G forces probably failed both the seat and the tail wheel retention structure.
You make a good point.
A failed seat frame could have tipped him over backwards as he zoomed into a climb to take care of the loose flopping elevator trim tab. Then, as he flailed about trying to sit back upright, his hand caught the tailwheel handle. With only seconds to get back upright and retake control, he was doomed.
That's what I was thinking until I saw the photo below. Looks like there is a panel right behind the seat, unless it's open below the shoulders.