It seems to be becoming the norm here to identify any shotgun in an underbarrel configuration as a Masterkey, even if it isn't one. I bring three examples, all listed as "KAC Masterkey" on their respective pages.
http://www.imfdb.org/w/images/a/a3/GRFS-MASTERKY.jpg
The UBS in Future Soldier is listed as a Masterkey, but the fact that it's clearly semi-auto and lacks M203 style mounting brackets means it isn't.
http://www.imfdb.org/w/images/0/0d/M...CMasterkey.jpg
The UBS in MGS4, while technically a Remington 870, lacks M203 style mounting brackets, instead attaching via an RIS mount, and as such isn't a Masterkey. To my knowledge, no such configuration of the Masterkey exists.
http://www.imfdb.org/w/images/6/61/CODBO3.jpg
The UBS in Black Ops similarly has no M203 style mounts in addition to having a full-length barrel, which no full-length version of the Masterkey exists to my knowledge. So hence it can't be a Masterkey either.
I feel we need to start being a bit more conservative when it comes to identifying underbarrel shotguns. We don't label every underbarrel grenade launcher on IMFDB as an M203, do we? A similar principle applies to using terms like "Masterkey style" in reference to underbarrel shotguns, as we don't refer to the likes of the AG-36 and GP30 as attaching "M203 style" to their respective parent rifles.