Man lots of people to thank for their sharing of knowledge and input. Edgar I think you have way more than enough background to adequately address my posed questions. In my opinion all PHD stands for is "Piled Higher and Deeper". The man is very smart though and lives metals. Last time I talked with him on a project involving a manufacturing process it took me two hours after the conversation looking through an old text book to figure out what he was talking about. You probably would have known immediately what he was talking about. My minor in Geology only takes me so far.
I get your points though. I also think you are right on target about the barrels and loads if the metal has been taken care of and was cleaned in its earlier days from any corrosive agents that could have been allowed to sit in the barrels. Reflecting on properties regarding copper coated lead that makes sense and with most modern shells the wad or shell cup I believe encases the load thru the barrels. The mass of load and its physical properties, or hardness probably matter most, plus throwing in velocity, etc. It definitely is not just the mass since steel has less mass than lead. Okay enough of that.
I tend to agree with point that biggest reason you want lighter loads cycling through these old guns, even if the barrels check out, is that the American Walnut used can become brittle with age and weakened if excessive oiling were to penetrate the wood near the wood metal interface, so forth and so forth. Its rare to see an old heavily used Parker 12 gauge that does not have stock issues. So now I understand the light load conundrum that slated for the charge discussion as it relates to pounding of the stock. So what is a light load.
I went to Polywad site and looked up their Vintage and Doublewide shells and they said something strange. Their site lists powder charge as 2 1/4 dram equivalent for 12 gauge. Checked the 16 Ga load, 2 1/4 dram equivalent, and it was the same for a 20 gauge. Boy that is helpful.
http://www.polywad.com/vintager.html
So as Edgar and Osthaus and others have/may eluded, the low pressure shells and loads is not just about the barrels but maybe more about the pounding of the stock, and heck who knows the delta and variables on that regarding the wood. I do know that on my gun the wood has not cracked around the lock. It does have come character dings and scratches but wood is tight and no signs of any stress there. Which brings up another question or thread. If you do not want to refinish the stock or wood, what is best way to care for these guns with aging Walnut other than limiting shooting 3 dram powder charges.
Researcher, love the handle, also appreciate your input and knowledge about the loads. It does go to show you the evolution of how guns and ammo change but essentially really stay about the same. Shot stringing, now that is an interesting topic, and not many folks nowadays even broach it. It clearly says on that old box of shells and loads from pic you provided showing "Super Short Shot Stringing". Bet you can't say that fast 3 times in a row. Time to get out the Physics text book. I think the English have it right. Square loads have a clear advantage for crossing moving targets. 3 dimensional thinking of shot travel and Newtonian Physics would be an interesting topic. I can't remember last time I have ever heard any talking head discuss about shotgun shooting and appropriately wade in on the shot string topic.