Do you have any background information on it? My first thought, given the ribbing, is that it's from a bomber or transport aircraft. Having that rounded frame piece "intrude" on the corner of the rectangular hatch is a distinctive feature. With the shape of the frame and the hinges on the inside, the hatch definitely opened inward.
Unfortunately, I'm not much of a hands-on kind of guy. Most of my knowledge is from pictures and this wouldn't be an area shown in a lot of "glamor" aircraft pictures.
I'm definitely going to be following this thread. Good luck!
-Irish