^^ and don't forget triggerplate actions as well - found on the John Dickson's and David Brown's
A sidelock has the lockwork situated on sideplates that sit right above the trigger. Most sidelocks are removable allowing easy repair to broken main springs and other parts. Many believe the triggers are better on the sidelock, while others feel the wood is weaker in the area where the locks are.
Noted gun writer Michael McIntosh is reputed to have put over 90,000 rounds through his AyA #2 which is a sidelock without issues, so take that for what it is worth.
Even without engraving, sidelocks tend to need more hand fitting and finishing, so they also cost more