It is my understanding that for the pigeon ring longer chambers were sometimes ordered from the factory, even at that time. The loads might not have increased but it allowed additional wadding, therefore better gas seal. I am aware of 3 chambers ordered around this time, perhaps a decade later. Remember, in the early 70s the only powder available was black and 3 1/2 drams of black with 1 1/4 oz shot left almost zero room for wadding in a 2 1/2 hull. If the gun you are referring to is relatively heavy, contrasted with a light game gun, then I suggest its possible the chambers are original.

When an old man dies a library burns to the ground. (Old African proverb)