Hi,
Keep in mind there is a difference between proof marks and gunmakers marks. Regardless, all of them were undoubtedly applied well after the barrel was made and hot. Certainly barrel makers may have stamped hot barrels with their ID stamps but it is very unlikely that the eventual gun makers did. For example, John Twigg had a barrel stamp but I suggest that it was never applied when the barrel was forged. Spanish gun makers used a constellation of stampings, some filled with gold, that must have been done well after the barrel was forged. Again, I think the key is soft iron versus modern mild steel.
dave