This is rather interesting. You know how there's one mini-square space after Darkrai? Well, I just completed Giratina, which is the last of the seven event bosses (for me, anyways). Well, now that empty mini-square area is filled! It's one of those box things, you know, the music items those seven bosses give you upon defeat. I dunno if it's anything important to the game, but it's there. (The mini-squares on the start-up screen is what I mean, just in case I wasn't clear.)
Did the progress-marker square appear after obtaining the seven instruments without the requirement of recruiting the legendary Pokémon guarding them? I was actually wondering whether recruiting the seven guardians was also necessary to trigger the progress-marker square to appear (because I ended up receiving the last instrument and recruiting the last guardian at the same time, I have no way of knowing which of my actions triggered the square to appear). Good job on noticing it, though; I haven't heard anyone else mention that well-hidden final progress-marker and was starting to wonder if I was somehow the only person who had unlocked it. XD
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I've noticed that the Golden Mask is
extremely effective at boosting the recruitment rate; it seems to have a significantly greater impact than the Friend Bow. While holding the Golden Mask, I can recruit almost any Pokémon after only one to three attempts, including fully evolved Pokémon like Charizard, Jolteon, and Rhyperior. The Golden Mask, because of its phenomenal boost to recruitment rate, seems to remove most of the luck aspect of recruiting, especially if the holder is Lv. 100. (Also, interestingly enough, the Golden Mask is one of the only items I've found that cannot be sold; the only other ones I've seen are the Enigma Part and Cryptic Tablet.)
Oh, and here's a slight correction to and clarification of my information on the Deoxys shades. The Speed Form Deoxys shade is found on the fifth floor of Dimensional Tower Heights, the Defense Form Deoxys shade is found on the sixth floor, and the Attack Form Deoxys shade is found on the seventh floor. None of the other floors have any unusual Pokémon.