Here is a nice remembrance of Polaroid's Positive/Negative films from The New York Times.
I used to occasionally shoot the Type 55 4X5 with an antique Graflex view camera. After the shot, the negative part of the film was placed in a solution of sodium sulfite in a special white plastic bucket made by Polaroid to clear it. Then it was washed and dried before printing. The ASA (ISO) was 50 and the quality of the negative was superb.
Polaroid ceased production of it's films last year.
Here is a shot I took with the Type 55 about 15 years ago and later scanned it.