The Giants issue a series of stickers every year similar to a card set under their "Giants Pride" brand - this brand is also seen in a BBM insert set and other souvenirs and marketing materials.
The 2015 stickers are pretty easy to identify due to the year located on two of the stickers found on the card-sized sticker sheet. There are also Giants and Tokyo text-logo stickers, the Yomiuri Giants interlocking YG logo, some fancy kanji that I don't recognize. The largest sticker features one player in front of a "scoreboard" featuring his jersey number. The backs are blank.
I'm not really sure how the Giants issue their annual cards/stickers, though the fan club may have something to do with it. I picked my copy up at one of the Tokyo-area card shops. Anyone with additional information on distribution is certainly welcome to comment below!