Here are a few pics of my Parkers...

[Bindlestiff]

If you can, measure the nominal bore size of the barrels. A 12 ga should measure .729 and an 11 ga should measure .750. If they measure something else don't be to surprised. Just one test to try.

[Bill Murphy]

Many early 12 gauge Parkers have bores as large or even larger than .750. The bore is not a test for 11 gauge. The test to determine an 11 gauge gun is in the chamber. Since the chamber can be enlarged, that is not a reliable test. The order book or stock book is the last word and they are not available. If your gun's chamber measures 11 gauge, then the assumption would be it was built that way.

[Gunner]

Bill, I'll bite what is the chamber size for an 11 gauge Parker?
Erick