Retired, so no longer in a position to verify, but it looks just like a King Air wing attachment bolt and barrel-nut. The clip on the nut is not a locking device; it's to hold the barrel-nut in place and correct orientation until the bolt mates it. The bore that the barrel-nut sits in is cylindrical, so its important that the bolt is not too long or it will damage the bore.
I say King Air but, of course, other aircraft may use a similar arrangement.