NPB made a big deal out of interleague play, just like the MLB did. And it's not uncommon for random oddball cards or entire sets to be produced for events. Put the two together, and you get this 60-card set.
The 2006 NPB interleague "season" was about three weeks long, from May 27th to June 18th. I haven't seen much about it in the past couple years, but generally teams "win" interleague play based on their record. There's no real benefit to it during the season other than just another boasting point... just like MLB. And of course, it's another excuse to create a bunch of limited special merchandise - there are pins, badges, towels, shirts, and, naturally, programs.
This program came from the Chiba Lotte Marines, which you can see prominently featured on the cover. The six player photos are all Marines players, and their team logo is the largest. In fact, next to the VS, in red letters, it says Chiba Lotte Marines, and in black letters, "version". So each team probably had their own interleague program. The yellow circle next to that talks about the free cards, basically saying that there are five different types to be found in this program, with two cards actually included inside.
Also, regarding the program itself, the first half or so of the program highlights the Marines and the players. There's a summary of the 2005 interleague series, and the last half (give or take) looks at each of the teams in the other league. The last page is a visual checklist of all sixty cards in the set, with details specifying that each program has two random cards from the five cards of that team. Again, this strengthens the idea that each team had their own interleague program; I'm not sure if you had to travel to each stadium (or order online) just to get them, or if each stadium had programs for all 12 teams.
Also also, at 1000 yen per program, and three programs at minimum required per team, it would cost you about $360 to build the set; you'd have one card left over from each team. This assumes perfect collation for each set of three programs. And of course, you'd have 36 programs, three for each team. What you'd do with the remaining two programs is anybody's guess.
Anyway, on to the cards.
This header card is blank-backed. The text basically reiterates what I've already mentioned - this is the 2006 Interleague Program's Original Baseball Cards. There are 5 different cards possible, with two inside. There are different programs for each team, and each team has their own five-card set which is found in their programs.
Backs are numbered in the top-right corner out of 60. While the BBM logo doesn't appear on the front of the card, as Engel mentions, their logo and copyright information is clearly visible on the back. Standard details found on BBM cards are also there - biographical information, awards and honors, and brief statistics. However, there are two lines for 2005, and the first line appears to be interleague statistics for that year.
I'm certainly happy to add these oddballs to my collection, but what I really need now is the Alex Ramirez card from this set. I doubt it's going to be easy to track down!