This post has been edited and republished after finding some new information. Thanks, NPBCardGuy, for inspiring some additional research.
From 2007 to 2010, Kellogg's cereal in Japan could be found with cards highlighting some of the best active MLB stars. I eventually decided to collect the entire set run of these cards, and that task was completed a couple years ago.
(Edited) The cards were included inside the boxes, one per box. There were chances to win prizes as well. At least, for 2008 and 2010. I haven't been able to confirm 2007 yet.
They were packaged inside little cellophane wrappers, one card per pack. None of the packs I've seen had any kind of marking on them, and probably just served to protect the cards in their display, or allow them to be attached to the cereal if the retailers so desired.
The 2007 set was produced by Upper Deck, had 10 cards, and had this design:
Moving on to the 2008 set, which was again produced by Upper Deck, and again totalling 10 cards:
Let's skip 2009 for a minute.
In 2010, Topps took over; this final set only had six cards:
Why did Topps issue the 2010 set? If you remember, Upper Deck lost its MLB license after the 2009 season, so Topps took over both the Kellogg's set and the Hideki Matsui Home Run Chronicles set.
What happened in 2009? I'm not sure. Over the past eight years, I've never come across a 2009 Kellogg's card. Searching Google yields no images. Other years pop up in Yahoo Japan Auctions, but no 2009. But I did discover something recently that might be the answer:
original post, were part of the box (if my translation is accurate). They do appear to be cut from the box.
If you're interested in checklists for the Kellogg's sets, I put them in this 2015 post (opens in new window). The checklists on that page include the 2009 set.
And now that I know what I'm looking for, I can start searching for the cereal box and/or rulers.