What does pinless mean?

Q: What is meant by the term “pinless” when applied to the description of a sidelock shotgun?

A: The traditional gunmaker’s term for a screw is a pin, so a pinless lockplate is one which has no visible screw heads, and therefore there is no interruption to the flow of the engraving pattern. The feature is usually only found on the most expensive guns.

If you think about this, you will wonder how on earth the lock is secured to the gun, because there surely must be a cross pin or screw, which would demand a screw head on one side of the gun? I was once faced with this dilemma when I took delivery of an extremely posh gun for test, and I eventually found a tiny trapdoor-like feature on one side, disguised in the engraving, which hid the screw. There may well be other means of securing the locks, but this struck me as a very elegant solution.