Here's an interesting piece of Majora's Mask History -
Apparently, in the original versions of the game the fourth transformation mask was the "Adult Link Mask", allowing Link to be restored to his older self without time travel. In the final game, it was changed to the Fierce Deity Mask.
Quite interesting, huh? I'm wondering if anyone can find any pics of when Link was wearing the mask, so we could see what he looked like and if there were any differences to the OoT Adult Link?
That would've been nice, I suppose. But I don't think, as superdemical said, I was missing anything in particular since it's just Adult Link...FD has a limited role, but at least it gave him a hell of a fan base, so something new was a nice plan.
I don't have much to say about this, other than this: I've found a site where they have some MM Beta screenshots. There's about 4-5 screenshots with Link weilding the Adult Link mask, but none of them show him actually using it.