Boxbirder,
How much pressure can a Parker safely handle?
How much pressure can a Parker reliably handle?
Those are not necessarily the same question.
Considerations toward either question are:
Is the gun in "as new" mechanical condition?
Is the action on face (tight)?
Are the barrels pitted?
Has any former pitting been reamed out of the barrels?
What is the barrel wall thickness measured at every inch from the breach? See link to Parker Fact:
http://parkerguns.org/pages/faq/BarrelThickness.htm
What frame size is the subject Parker?
Are the barrels fluid steel, laminated steel, twist, or Damascus?
In determining if a 100 year old gun is safe to shoot at what pressure, i.e. for 12 gauge a 6000 PSI low pressure load or 11,500 PSI SAAMI Max pressure (reference ANSI/SAAMI Z299.2-1992), all of the above and probably factors more need to be considered. That is why the PGCA doesn't boast about how much pressure a Parker can handle. An "as new" Parker on a larger frame with overall thick barrels might be able to shoot modern loads. A similar Parker on a #1 frame with laminated steel barrels, that may have been reamed, might injure the shooter with modern high pressure loads. This is why the PGCA recommends having one's Parker examined by an experienced double-gunsmith.
Mark