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.
Purdey was a pupil of Joseph Manton (along with Boss, Lang, Lancaster and Greener (senior)). All built 'best' guns when they went on to set up shop on their own - as you might expect from Manton's influence.
As so often happens - cheap copies, the equivalent of today's Rolex copies I suppose, were made, usually overseas from the UK and often with names designed to mislead by being slightly mis-spelled. Usually these were of obviously poor quality, but a few were rather better.
Purdeys, like all except Boss have also built and sold lower grades under their own name, but always still of good quality, just not 'best' quality. Boss lesser grades were sold under the name ''Robertson', the owner of the Boss business at the time. They are also of good quality.
All London makers used various outsources (including some in Birmingham) at different times, but these were also the finest craftsmen around and the 'best' quality was maintained.