Thanks Ian, I had found the 1872 reference to #3791 but had not thought of your "use number" comment. Am I correct in that a use number would be like a contract permission of using the Anson patent?
So we now know the gun was made after 1872.
Hepplestone made guns at the address (25 Shudehill, Manchester) on the barrel from 1867 to 1910. So my rifle was made between 1872 and 1910. A broad range. If anyone has any more information that refines the dates I will appreciate it.