When it comes to trading cards in Japan, gum doesn't get older than this.
Coming in around just 1-1/16" wide and 1-7/16" tall, give or take, these are somewhere around the size of those tiny Topps Micro cards from around 1992. At least five series were issued, or at least types of cards were designed.
Type I is the version you see here - I think. This is the easiest to distinguish because "Asayama Fusen Gum" is printed vertically (that's the white box near the top). However, the example in Engel's book has two white boxes, with the player's name appearing in the second box as well. Furthermore, none of the cards listed is the one I have. That doesn't mean it doesn't come from this series, because there are possibly uncataloged cards in this set.
Type II has the Asayama Fusen Gum box at the bottom, horizontally, with a border around that box.
Type III's horizontal box is also at the bottom, but oval-shaped (possibly without a border?).
Type IV has the text at the bottom, but without a box.
Finally, Type V features San Francisco Seals players exclusively, and there is a white box at the bottom without a border.
The cards are all blank-backed and quite rare, receiving the designation "R5" - the rarest scarcity of known copies. As such, they carry a premium in better condition. I got the card above from a seller who listed several different singles from many of the series, but I was outbid on the other auctions. Still, this is a pretty cool little (really little) card to have!