Topsun

Top-Seika is a publishing company that signed a contract with Shogakukan, which held the rights to the Pokémon anime, in 1997. They subsequently began releasing sets of cards that were packaged with candy, often gum.

The first set was released in March 1997, 2 cards and 2 pieces of green apple-flavored gum for 60 yen. This first run features a blue back, with the first printing of several Pokémon missing a Pokédex number (at least 50 of the 151), referred to as no-number error cards. A reprint released in May 1997 with a green back. Of note, the holographic cards in both sets were actually stickers.

They would also release cardless Pokémon-themed gum, grape and lemon flavored, on January and April 1997, respectfully.

These cards are often touted as being the first Pokémon card set, due to the 1995 copyright on the back. They are, however, newer than the Pokémon Trading Card Game, which released in 1996.

Top-Seika would release several other collectable card and sticker sets, though I've not found any comprehensive list. That said, dating is often much easier, as the copyright dates on the back of newer cards seem to be fully accurate to the year of printing. There's a lesser method of confirming dates via the company name printed on the card backs. Cards printed before Fall 1998 have the company name as Topsun; afterwards it's Top.

An incomplete list of Topsun cards:

Topsun (No-Number Error)	March 1997
Topsun (Blue Back)	March 1997
Topsun (Green Back)	May 1997
Gold-Silver-Crystal VS Cards	2000	Brown border
Advanced Generation VS Cards	2004	Pink border
Advanced Generation/Diamond Pearl VS Cards		Green border
Diamond-Pearl VS Cards		Blue border
Diamond-Pearl VS Cards (Borderless)
Black-White VS Cards		Layered cards, one transparent with the attacking Pokemon, which could be overlaid a second card with the receiving Pokemon on a solid background
XY VS Cards		Layered cards, one transparent with the attacking Pokemon, which could be overlaid a second card with the receiving Pokemon on a solid background