From Greener, the gun and its development p.301 14/1 is .701
However another article I found online:
With a double gun, even one in original condition, the two barrel sizes may not be stamped up the same. With one that has been worked on in the bores and reproofed they may be quite different and even carry proof marks from a later period marked to another system. A very early breechloader marked up 12B 13M means that the main bore is gauged at 12 and the muzzle is gauged at 13, so it is choked. Not for ball was also an old way of signifying a choked barrel.
https://www.shootinguk.co.uk/answers/technical-answers/explaining-shotgun-bore-4610. Bottom line: I cannot answer the meaning of the 14/1 with 100% confidence without being able to measure the bores and they may have changed enough over time to blur an answer.
