I don't really know anything about this card, except that it is huge, roughly the size of a modern comic book! I'm not sure if it is a reprint of a previous card, or an insert from a book. The seller had one Gamera and one Ultraman card of simliar size that I didn't buy.
Perforations on the left side of the card are a clue. I have scans on my hard drive of a 1971 book of "Godzilla and Other Monsters" (which does include Ultraman and Gamera) but it doesn't look like a book of giant postcards--however this card is definitely a page in that book:
Great art of Godzilla fighting Ebirah though!