I'll venture a guess.
Perhaps the gun is a muzzle loader.
The mainspring is compressed by pushing in on the push-rod, and once the sear has caught, the return spring returns the push-rod back to it's forward position.
A projectile is then inserted from the muzzle.
If a ball is used for ammunition, it may perhaps use a small spring retainer (similar to some BB guns) to hold the ball from rolling out the muzzle should the gun be pointed downwards.