There was a loooong thread dealing with this 'Masterkey'. And there are still a few issues to deal with.
Barrel, it has to be a factory manufactured barrel, if less than 18", as the one in the pic appears to be.
As mentioned, the T.O.L. has to be greater than 26". The 'problem' of using the shotgun alone and saying that it could not be used without the "host" AR15 was sort of answered. As with all things legal, would probably take a court challange to get a definate interpretation. I believe if the stock 'stud' was removed from the 870 receiver, thereby eliminating the installation of a butt stock would help the cause saying the host AR15 would provide the T.O.L. of the entire firearm.
And again as already mentioned, and mentioned in the old thread, if you marry the 870 up to a restricted host gun, AR15, then it becomes restricted anyway..
The old thread was trying to deal with having the 'Masterkey' on a non-restricted firearm, and therefore even with the T.O.L. of the 870 being less than 26", it was being discussed using the host gun to be included in the T.O.L. of the non-restricted gun and the 'Masterkey'.