Al, I expect, (like on our boat of similar vintage), it will be one of those locks recessed into the door leading edge, actuated by a large and clumsy old fashioned type key, (usually lost because they are so annoyingly large and clumsy, & just don't fit well on keyring - or you sound like a jailer walking along), and it activates a sort of bronze or steel tooth type of thing which revolves out and over and down from the door edge, and is meant to catch over the edge of a recessed metal-rimmed aperture in the door jamb, but which often doesn't if there has been any significant deformation of door or frame. Let's see if I'm right...