From an article written by Ross Seyfried in the Autumn 1996 Double Gun Journal titled Transitions Double Rifles from Black Powder to Smokeless.
He references a Holland and Holland .450 Royal Hammerless like mine except mine has animals engraved all over the action and locks.
Here is is quote on the "lumps" on top of the breech.
"Atop the standing breech are two bumps. These protrusions are clearly without purpose beyond ornament, unless you attach purpose to clinging nostalgia. The bumps are vestigial HAMMERS! Alas, shooters had a very difficult time letting go of this most elegant form of firearm"