Polly Mary. The change of M for P in pet names is by no means rare; e.g. -
Margaret. Maggie or Meggy, becomes Peggie, and Pegg or Peg.
Martha. Matty becomes Patty.
Mary. Molly becomes Polly or Poll.
Here we see another change by no means unusual- that of r into l or ll. Similarly, Sarah becomes Sally;
Dorothea, Dora, becomes Dolly; Harry, Hal.