I have finally been able to id this lock plate thanks to a kind fella at the British Military Forums.
From Robert Brooker's book, British Military Pistols. There is an image in the book of a pistol with this same lockplate, even with the Tower over H.Gill. Since it is in a book , its probably copyrighted so I won't post it here
It is a late 1830's, Commercial New Land Pattern Pistol for Yeomanry Troops.
What are Yeomanry troops you ask.....
From wiki:
During the first half of the nineteenth century, Yeomanry Regiments were used extensively in support of the civil authority to quell riots and civil disturbances, including the Peterloo Massacre; as police forces were created and took over this role, the Yeomanry concentrated on local defence. In 1827, it was decided for financial reasons to reduce the number of yeomanry regiments, disbanding those that had not been required to assist the civil power over the previous decade. A number of independent troops were also dissolved. Following these reductions, the yeomanry establishment was fixed at 22 corps (regiments) receiving allowances and a further 16 serving without pay.
During the 1830s, the number of yeomanry units fluctuated, reflecting the level of civil unrest in any particular region at any particular time.