Better inspecting the image, it looks that in the left side we have a cut cable and not a socket... so probably last owner cut a PC connector form an old sync cable to use it as a cap to protect the PC socket from taking dirt, dust, etc.
With DSLRs we use this:
Last user could have used an smaller connector for the protecting cap effect, but he used what he had at hand... Well, now Darryl knows that the former owner took care of his equipment, this is always good.
-) best solution would be one of those caps delivered with more modern SLRs. Not very common in rummage boxes... One could have even glued a string to it to not get it lost.
-) second best solution would have been what you just described.
-) the thing in your first picture though is a hermaprodite plug, I never have seen before, and not a cut-off PC connector, cut off from a sync cable.