Most shooters recognise a boxlock and can tell it from a sidelock. A hammer gun is a clearly different creature but a trigger plate gun is harder to spot unless you look a little closer.
The idea goes right back to the period in the 1870s, when gunmakers were trying to work ot the best way of getting the hammers inside the gun in order to make a ‘hammerless’ profile outside. The obvious way was to simply make the hammers smaller and mount them on the inside of the lock plates (which is basically what the Holland & Holland ‘Royal’ is). This is the most common solution too. The Anson & Deeley patent of 1875 was a completely different idea. This created a ‘box’ of steel as the action, machined out slots for internal hammers and mainsprings, the sears fitted in underneath and a bottom plate hid the workings from view.
The ‘third way’ was to make a bigger, wider, trigger plate and build the lockwork onto plates mounted upon it and inserted into the action from below. There are lots of them, Horation Phillips had some success with his, Stephen Grant being the most commonly encountered maker of them. Several lever -cocking patents were quite successful and the Scottish maker Dickson became very well known for his ’round action’, still made today. MacNaughton also enjoyed a good reputation with his lovely looking ‘Edinburgh Gun’, also a trigger plate mechanism. The modern British trigger plate lock is best exemplified by David MacKay Brown of Bothwell but the idea persists in continental interpretations in the form of modern Purdey ‘Sporter’ models.