Phil,
Thanks for your post. I will try to help with your question.
As you know, the internal diameters of the bores are the ultimate determining factor. The bore diameter for a perfect 11-ga shotgun is 0.751". If the gun is a 12-ga, the bore is 0.729" and 0.775" for a 10 bore. A 12-bore gun with barrels that are very badly pitted or have been honed could look like an 11-bore, but pitting or honing would not alter the chambers unless they were intentionally altered to try to make a fake 11-ga gun. I have not heard of any instances of that being done. Your gun may be a 10-ga that was chambered for the 10-B brass shells, or it may be an 11-ga. The only way to answer this question is to use an internal micrometer to measure the dimensions of the bores and inspect them to make sure they have not been honed. If they are full length tapered bores, the diameter must be measured at 9" from the breech.
My measurements of the Prototype Parker S/N 06 at the Meriden Historical Society (with the help of Charlie Herzog) and several of the Bill Furnish early Parkers reveal that they were able to make their 12-ga bores precisely .729" and their 11-ga guns precisely .751" diameter from the very beginning. They were always perfectionists at Parker Bros.! I hope this helps.
Can you post some images of your Parker? Is it a Back Action Underlifter? And what are the barrels - laminated or Damascus?
Richard
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