This is a picture I have that I grabbed somewhere. If I remembered where I got it, I'd note the source.
Originally it was designed to clear the action automatically if a shell slipped past the shell stop and it was a reliability feature. However, people discovered that it could be used to load an extra shell on the carrier which allowed the unscrupulous hunters to have their gun plugged to only load 3 shots when done normally, during migratory bird seasons, but effectively they have 4 shots when loading one on the carrier.
I actually figured this one out on my own SBE circa 1994. It is easy to do on that era's SBE, but the shorter chambered guns are more tricky with the bolt position.
Anyway, the picture below of the bolts show the M1 and M2 bolt differences, but the ability to ghost load or not boils down to the notch towards the tail end of the bolt. If there is no notch, it should allow ghost load. From what I can tell, almost all Benelli USA models no longer have that capability, and almost all HK imported ones do (unless either has had the bolts swapped). Provided they haven't added any other differences recently, all one would need to do is weld or otherwise fill that notch. With that said, there's really little practical use outside of the few hunters trying to cheat the system. All other roles just add an extended magazine. It is faster and easier to load though with some practice one can ghost load in a few seconds. Admittedly, I did use this in my starting load with my SBE when toying around in 3 gun because I didn't want to modify the gun as it was my main "do-all" hunting shotgun. After that first loading, I just loaded the usual amount. I eventually bought an M1 and while it has the ability to ghost load, I just use the full length tube even for the first magazine full.
I have a pair of the older HK imported M3T's folders which can both ghost load, but other than trying it once on each to verify the correct bolts were in them and it hadn't been swapped, I haven't done that since.