Traditions die hard, especially in the gun world. When hammerless shotguns like this came on the scene in the 1870s, shooters had already been using hammer guns for hundreds of years. A lot of these people were unwilling to give them up.
Before the hammerless era, shooters used external hammers to cock their guns. They also used them as visual indicators: cocked hammers, cocked gun. External hammers also gave shooters the freedom to decock a gun, so if a shot wasn’t taken the gun could be returned to this “safe” position again. A gun could also be carried in the decocked position and then cocked right before the shot (like when a grouse flushed).
The new sidelocks and boxlocks coming onto the scene had none of these advantages. This made some shooters reluctant to jump into the hammerless shotgun future. So what did some makers do? In one instance, James Purdey & Sons simply combined the best of the old and the new. They made this pair of 12 gauge double barrels around 1879 and they’re part past and part future.
While they’re mostly modern (a barrel-cocking centerfire with internal hammers), the maker kept part of old school by adding “hammers” to the outside of the locks. These “hammers” are really cocked indicators. When the gun is ready to fire, they’ll tip back into the rear position and give the shooter that visual indication he expects. If the wants to make the gun “safe”, he can drop these “hammer” into their rest position and decock the gun. Interestingly, both of these Purdeys were made without safeties. You can see how a top safety was added to one of the guns at a later date, maybe to make the gun more handy for walk-up/rough shooting.