Wiktionary
flat lock
n. (context construction English) a method of interlocking metal panels in which one panel edge is folded back on top of itself and the other panel is folded under, after which the two panels are hooked together.
Usage examples of "flat lock".
They fired from the decks and from the upperworks, and Sharpe fired with them, using one of the Sea Service muskets which was similar to the weapon he had fired when he was a private, but with a slightly shorter barrel and an old-fashioned flat lock which looked crude, but, as Llewellyn explained, was easier to repair at sea.