Wow, I have been in this gals shop years and years ago. I go to Dorset fairly regularly but haven't tried to find her shop again. It was amazing the one time I was there. Now that I know she is still open, I will have to check her out again.
Regarding how it was made, I would bet the single contrasting square was appliqued on each block corner the same time the pumpkin seed shape was appliqued. Once the blocks were assembled they formed the 4P unit.