Red LegExactly! - a drop lock then. Congratulations on owning one of the finest guns ever produced by human hands. I don't suppose that shop owner has the original oak and leather case tucked away in a back room somewhere? Depending on condition, it alone could be worth more than you paid for that wonderful gun. Game Bore are great loads. I use them a lot here (almost exclusively in older guns like yours). I would restrict that gun 1 1/8 ounce loads, and with American size No. 6 shot you could cleanly take any pheasant or duck that ever flew.
Interestingly enough , this gun is one of a pair. Somebody purchased the other shot-gun without the case. I think that I will purchase the case. It will make the gun look complete. I am an avid fan of game bore cartridges . I use their Buffalo SG cartridge in the lower barrel of my Beretta 12 bore ( of course , no sensible person would want to use an SG cartridge in this Westley Richards piece ) .
I see that you are advising not to use number 4 cartridges and to stay with number 6. I shall stay with number 6 then. Bangladesh importing Game Bore cartridges was one of the widest decisions BSF had made.