Hi Scott,
I am not familiar with the lock in question but it usually means a pan that has a raised lip, which acts as a gutter to keep water from getting into the pan. Some locks had slots or drain holes cut around the raised lip to drain water. On other locks the pan was a pedestal separate from the fence and bolster for the frizzen preventing water from draining into the pan. All of those features worked to some extent but black powder is so hygroscopic that it eventually absorbs moisture from the environment anyway.
dave