Thanks for your replies.
As best I can date the gun going on others I've seen listed with their dates, mine is c1844 - well after John's death.
I had the locks off last night for a look (no qualms about using any old turnscrew for that; someone's already had a go before me - the slot is buggered, but the rest are good). They are initialled "GM"; may I assume that they were George Manton's initials?
I like it the more I look at it.
RG