Just my opinion, but I'd have to agree that the gun wasn't made by Purdey, but is marked for Purdey's patent of 1863. The 748 is the use number that was issued when the maker purchased the right to use the patent. Each individual non Purdey gun would have their own use number. A Purdey made gun might have the patent name and number, but no use number since they owned the rights. If 748 was a Purdey serial number it would have been made around 1812. I think the patent lasted 17 years, so this was probably made before 1880 after which they could have copied the patent without buying the right. If you google the Purdey patent of 1863 you'll probably find a few by other makers, Blanche for one, that mention they're made on the Purdey patent under bolt (or bolt) of 1863. Proof mark says English made. A continental knock off wouldn't have paid for the patent rights.
As far as the engraved name goes, most Purdeys of this vintage would have had "James Purdey" engraved on the side plates. My Purdey hammer gun was made in 1877 and just has James Purdey on the plates. The barrel has the name and address, but no "and Sons". Purdey didn't add the "and Sons" until around 1881 or 1882. I don't recall off hand the year. Kind of odd since several generations of sons had already worked with their fathers by then. Some guns that were ordered without or with minimal engraving just had Purdey on them.
It is a nice looking action. I've got a 20 bore Osborne hammer gun action that I use as a paper weight on my desk.