Were Purdeys of that vintage Birmingham or London-sourced guns? Wasn't Purdey, like many others, at least initially, sellers of sourced guns as well as Makers? Wasn't it made long before Purdey had patented his own sidelock action, as well as before he bought the Beesley-patented self-opening action that he's used ever since?
James Purdey only built best guns in house from the moment he set up shop. There was a time, around the turn of the last century that to keep up with demand of lower grade guns, Purdey did bring in semi-finished guns from the Birmingham trade to finish in house. These guns were always marked as to what grade they were (not best), and their production ran parallel to Purdey's normal output. This was long after the percussion period.
This gun does not appear to be a genuine Purdey. While the name is hard to read, it looks like it is one letter short. I think I see 5 characters, not 6 as in Purdey. I think I read "Purdy" which is the classic spelling on a Belgian fake.
Secondly, unless the barrels are a replacement, they are not of best quality. There are no blowout plugs.
Finally, the engraving isn't close to the quality of work that Purdey guns have in that time period.