identification of parts B-17 ?

This may be the hinge on one of the escape hatches (the tail gunner's hatch or the navigator's hatch)  or even the main entrance door at the rear of the plane.   It appears to have a  removable hinge pin.  The pins on a door or  hatch  were attached to a wire  or cable which ran to a  single "D ring".    If the bailout order was given, the D ring could be pulled which in turn pulled out all  the hinge pins  on the door.  The door/hatch  could then be kicked out or  jettisoned which allowed the men to bail out.  I don't think any of the current flying B-17s would  probably have that type of door/hatch so you would need to find an old photo to verify. 

Hmmm,  I am now having second thoughts on this.  Below is a picture ( you can click on the picture to zoom in) which shows the rear door (removed) and the the tail gunner's hatch (under the stabilizer).   The hinge mechanism on both these doors (and on pictures I have found of the front hatch) is actually a flat metal circular arc that goes through a slot next to the door opening.    It actually is attached to the door on the outside of the plane.   You can see the slots just to the right of the removed door.     On the inside, I would think the arc would just fit into the other "half" of the hinge and a hole would have been drilled through the arc to allow the pin to pass through.   There seem to be two  "metal arcs"  per door.   When the pins are pulled, I would assume the door with the two "hinge" arcs gets jettisoned, the arcs would just slip out through the slots.  
 
On the photo that damdar has above, there seems to be "too much stuff" on the inside part to allow it to slide easily out of the slot.   So...., are we looking at bulkhead door hinges,  bomb bay door hinges, or something else?  His picture does seem to have the "removable" pin, so what other "doors" on a B-17 would be set up that way?  Would a bulkhead door ( there was a door between the radio room and the back  of the ship, between the radio room and the bomb bay and between the bomb bay and the front of the ship) have this type of hinge?
 
http://media.photobucket.com/image/B-17%20tail%20gunner%20hatch/ernst_rohr/B-17righttail.jpg
 
Here is a photo of the  rear door swung open.  It gives a good view of the hinge "arcs".
http://farm5.static.flickr.com/4041/4646345070_ced53d232e.jpg

Just a small detail here: the curved M in the Aircraft Inspection Stamp is a proof that this particular part was made by Murray Corporation of America in Detroit (also known as Murray Body Corporation). They were contracted to manufacture parts for B-17 and B-29. I don't know what did the company deliver to Boeing (wings and nacelles are listed with the company name here: http://www.heritageresearch.com/ourlibrary/databases/wwii/authorized/michigan.htm ) , but as per the same link they also supplied parts for P-61 and P-47 (eventually with the same stamp).