Parker 51 Aerometric Sac
I have a Parker 51 Flighter silver fountain pen left from my father. From outside appearances, the pen is in remarkably good shape. Refurbishing the pen, I was surprised that the sac was clear plastic, also known as a "pli-glass" sac. The original sac was intact, but was stained black by ink. Both the original sac and the replacement sac loosely fit over the base. I suppose that is how the Parker 51 sacs are designed. Shellaced the new sac in place, and it's back in business.