Here are the Remington era chamber specifications from the Specification sheets that were reproduced in
The Parker Story.
Remington era Chamber Specifications.jpg
No one can tell you over the internet what ammunition may or may not be safe in a given gun. But, a 12-gauge 2-frame Parker in solid condition should handle 1 1/4-ounce progressive burning smokeless powder, high velocity lead loads and has probably in its life digested plenty of Super-X or Remington Express.