I can confirm that Douglas did not "proof" test their barrels.
They may have test fired them, but to actually "proof test" the barrel must be sent to an actual, official, proofing house. The term "proof" and "proofing" are legal and technical terms that are very specific. For example Winchester tests their modern rifles but has to call it "winproofing" because they are not submitting the items to an actual independent proofing house. So when a custom muzzleloader builder says they "we proof every gun we make" ..., NOPE, they are test firing that gun.
Don't misunderstand, one can do some extensive testing, perhaps even exceed the testing done on a black powder barrel at one of the official proofing houses..., but it's still not "proofed".
LD
I misspoke, I was actually referring to and accuracy test, or run out location. I have a book somewhere that Douglas would stamp their barrels in certain way to indicate it.