I looked up the Vultee Vengeance on the line to get a bit of casual information on its history and immediately began to realize that the part shown simply did not fit...
The same image turned up on a flickr site and this comment
clarified things:

"I believe this transparency must have migrated from another file - the part being assembled in the nose door for a Consolidated C-87 'Liberator Express,' probably taken at Consolidated's Fort Worth plant."

The shape seems right for C-87...I suspect the "key" is perhaps the handle of a latch allowing the door to be opened below that little panel...or maybe an actual key on the hypotheticl latch...the AAF wouldn't want the natives to steal the spare tire!

On the lower left, something looks like a lock with a key inside.

My grandmother and grandfather worked there during the war. Grandma worked during the day as a secretary and Grandpa worked in the factory at night. This gave my mom plenty of time to sneak off with my father, riding on his old Indian Chief motorcycle for a few clandestine dates. Would love to see more pictures from Vultee.

Built in the U.S.A. never used in combat by the U.S. many sold to Britain, Australia, India, France, Brazil, ended up as a target-tug.

Those are called CLECOS. They act as temporary rivets and can be removed prior to final assembly.

They worked so hard making these pieces that their pneumatic riveters didn't even require air lines to run!