Door hardware.
Brad, If it's anything like what I have on our 1979 E31, it's simply a latch consisting of a piece of flat bar made of brass with a notch cut out at one end and a hole at the other. The hole allows a screw or pin to pass through that acts as a pivot and the notch at the other end engages a plate or another pin, can't remember. It's design is utterly simple yet effective. If I had to guess, I'd say that the brass is 1" wide and as long as needed to do the job. With a drill or even better yet, drill press, you could make one yourself in under 10 minutes if that. Once installed, one need simply lift the bar through the large opening with a finger, thereby unlatching the bar, etc. Make your own and nobody will be the wiser. I'm not at the boat but just looked at a photo of that door and the opening in it is identical to the image you posted of your door. Hope this helps, Glyn Judson, E31 hull #55, MArina del Rey, CA