I just heard from Purdey and they said the number refers to a 16 bore muzzle loading shot gun made in 1829.

After raising my voice little on phone the dealer who offered me the gun was able to read "Lyon" on the rib so it might be one of those made by "Lyon & Lyon" and James Lyon did work for Purdey

Thank you .500 Nitro alot of time and money saved!