The records also show that the rifle was built in 1886, but it was actually sold in 1891.
I think that the "actually sold in 1891" is the key.
If you buy a new Purdey gun today that is in stock the information says "chokes cut as required" or "chokes as required". All of their "in stock" guns are extra full allowing them to cut the chokes to their customers desire.
I'll bet that this gun was not regulated until sold. They would most likely have waited until the customer told them the desired charge and bullet weight before regulating the rifle. When it was sold in 1891 Cordite was in use and the customer most likely wanted the gun regulated for his bullet weight using Cordite.