It looks like the bolt is broken off.   I would suggest drilling it out starting with a small drill then progressively larger...   You might try heating it then using an easy out but I would be careful because of the possible sheet metal distortion.... So be careful there... 

By the way, if you don't have a torch, look into the induction heaters....  They create a localized hotspot and might work well in this situation.

Mike

I agree.  It does appear the bolt has snapped off.  There should be two bolts on each gate side that secure a trunnion, see attached photos, that hold the tailgate in place.

Nick,

Your bed should be 48 inches wide. The narrower bed was used for '37 through '39. The '39 uses a fender that is similar to the '40/'46 fender except that it is wider so the bed/fender combination is the same width, overall, after the bed was widened for 1940.

Mike

Well shucks, Mike!

Here comes a complication!

The tag on my cowl indicates that the truck was built in December of 1939 and my bed is 45 5/8" x 78 1/4".

Is it possible that some of the early 1940 KC trucks came with the older style narrower bed? Or is it a reality that my truck is a mongrel...1/2 1939 and 1/2 '40? 

Will I be facing some fitment problems when I try to marry up the running boards and the front and rear fenders?

Nick

You will have fitment problems if your fenders and bed are not matched. I doubt that any overlap of "left over" parts took place to explain your situation.

Mike

Phooey! That's not what I wanted to hear.

Now I have to find someone with a good later bed but needs one that is the older, narrower type.

It's too bad because the bed I have (with the exception of those two broken bolts) has been gone over by a previous owner and is in very good shape!

Well, one step at a time!

I have recently dealt with this bolt problem. Your bolt is broken off and does have a captured square nut behind it. I guess the original bolt had a wide head like a carriage bolt with the screwdriver slot in it. You may need to modify a carriage bolt to get that look. Removing the rusted broken bolt is a very common undertaking in this hobby. When assembling the hinge/tailgate assembly, try to "dry fit" the whole thing before committing to tightening bolts. I made a mistake in thinking it would all go back together with new hinges, but with wear & deformity of the bed and tailgate, it didn't. I ended up drilling some new holes that I didn't want to. 

Jake