I don’t think they are compressors for an easy/assisted opening system (is there any type of assisted opening system fitted to either of the two guns illustrated anyway?).

They are retractors for the cocking rods which run from the knuckle to the front face of the tumber.

When the action is cocked the cocking dog on the forend pushes the cocking rod back against the tumbler (to cock it). Normally the rod would be left resting against the tumbler & have to be pushed forward as the tumbler fell to strike the firing pin (thus reducing the blow on the pin). The hidden ends of these buttons have angled ‘blades’ which protrude into a slot on the cocking rod – this slot has a matching slope to one end. As the button is pushed down the angled surfaces ensure the rod is pushed forward clear of the swing of the tumbler.

It’s originally a Perkes patented feature which is most often found on some of the back action sidelocks made by Scott. It was generally used on non-ejector guns but can be found on a few ejector models (due to differences in the lockwork on some ejector guns the buttons are actually redundant). I think that Perkes concern about reduction in tumbler stricken efficiency was proved to be unfunded so use of the feature gradually faded away. Scott themselves didn’t use the system on most of their back action guns. Blanch seemed to specifiy it for some of their Scott/W&S sourced guns into the 1900’s.

Regards
Webley