The GAF tri-fold format was distributed in the European market and was roughly contemporaneous with the North American G3 through G6 variants.
The tri-fold style dispensed not only with an exterior envelope (as did the packet-book format) but with a booklet as well. The package consisted of a pamphlet printed on coated cardstock with full-color panels serving as front and back covers; the interior comprised only text, with captions for the packet's 21 images provided in multiple languages.
In some (perhaps earlier) examples the three reels were stored in a paper sleeve glued to one of the pamphlet's six panels, but more commonly the reels were shipped loose inside the pamphlet; in either case the pamphlet was enclosed in a heavyweight clear plastic sleeve.
The front of the booklet displayed a View-Master logo similar to that on previous envelopes and packet-books of Belgian issue, a narrow rectangular bar featuring the GAF logo and the words "VIEW-MASTER 21 Stereo Pictures" (or their equivalent in other languages) in white curvilinear letters against a black background.
The packet number appeared in the lower left corner of the back cover.
It was also quite common for tri-fold packets to carry a secondary number representing its
sequence in a series of packets devoted to a subject (e.g. "Belgium Series - 6"
in the example above).