Kingpin adapters - B&W Turnover, Gooseneck coupler kingpin replacement
 

I am looking at some different trailer and hitch configurations and am curious how the gooseneck kingpin adapters work - the ones where you swap the ball coupler with a kingpin. I know there are the kingpin + ball plate adapters, I knew someone that did that with a semi tractor. Here is an example of what I am talking about:
https://www.pjtrailers.com/options/kingpin-adapter/

I am also a bit baffled as to what the purpose is of the B&W kingpin adapter for the Turnover system. They have a few others that don't make sense to me also - the reverse gooseneck ball coupler, eyelet, ect. Of all of them the eyelet makes the most sense for an attachment point of a tow strap/chain, but not as much as a conventional class III-V rear mounted receiver. Why would you want the reverse end of a trailer coupling method to be mounted in the bed? I can't see someone inverting their 5th wheel hitch for any possible reason, for example. For what I am talking about see the bottom of page 5 of B&W's PDF catalog listed here:
http://www.bwtrailerhitches.com/site...BW-Catalog.pdf

Generally gooseneck trailers are made with a 2 piece tube in a tube (round or square) adjustable hitch. The larger open tube is welded to the trailer and the smaller with the coupler attached slips inside it. This is adjustable with a set of holes for up and down height adjustment. If you want to pull a gooseneck trailer with a truck that has a fifth wheel you can usually just remove the gooseneck coupler and put in the king pin one you linked to.

These are all made for odd ball trailers.

inverted gooseneck ball coupler is used with the ball on the trailer and is secured with a pin in the coupler on the truck.
Eyelet is similar to a gooseneck but couples differently I think. I have seen these at farm auctions on older trucks with home made trailers.
The kingpin one for the truck has a small fifth wheel head on the trailer upside down. It is actually what the binkley fifth wheel head that the air ride trailer saver brand fifth wheel is made for.

My assumption is that some of this is for a theft deterrent. Pick something goofy and it is harder for someone to borrow or steal.