It turns out not to be so easy to identify the maker of a random piece of play equipment using just a captionless photo from a 1968 craft magazine, and not just because I didn't know how to say "slide" in various foreign languages. It was the German [Kinderrutsche or Rutsche] and French [une glissiere] that led me to Beltzig, who, in true Hasselhoffian form, has gone on to become the most famous playground designer in Germany. But first, his first slide.
From Walter Papst to Luigi Colani to Gunther Beltzig and his brother Carl, Germany emerged as something of a hotbed of innovation for plastic and fiberglass children's furniture design in the 1960's. [Yeah, this whole thing is basically Hasselhoff level of esoteric history, so let's grade it all on a curve.]
In 1966, the Gebruder Beltzig designed a collection of single-piece, molded fiberglass play furniture, which they intended for both indoor and outdoor use. There was a play table and chairs, as well as a see-saw, a rocking tub-like thing, and the incredible, organically shaped slide. Everything was light and strong enough for kids to cart it around themselves. Form Magazine quickly took notice and began evangelizing for more playground equipment and kids furniture out of this perfect material.
It's not mentioned in Form, but one gallery in Germany selling a play table says it's part of the Beltzig's "Poly-Bel" series. But other sources say they only produced a small batch--one gallery with three chairs for sale says only 150 were made. One thing seems certain, though: it doesn't leave much of a trace in the market, and while there are a few chairs and tables around, I've only found one non-archival picture of a slide, in a private collection of children's furniture in Cologne.
Even if the Poly-Bel [sic] series wasn't a big commercial success, the Beltzigs kept on making ever more elaborately shaped fiberglass play equipment. This single-piece merry-go-round is from the early 1970's:
And that same dealer, Froelichladen, shows a picture of a most excellent and awesome picnic table, called the Auberge. It says it's a limited edition. Coincidentally, it's also the same color as the Auberge which didn't sell in 2006 for EUR8-10,000 at Quittenbaum:
Too bad that thing's adult-size--185cm across--because it'd be awesome for a kid. Which is probably what the Beltzigs were thinking when they made the similar-looking, kid-sized Filius play table in 1974. This one's in the Vitra furniture collection, on perennial tour with their Kid Size exhibition. [Beltzig contributed an essay to the catalogue, too, btw.] There are more shown on Beltzig's studio site, along with some even bigger, crazier fiberglass mountains and playgrounds.
Not that 1974 was a particularly good year to be making anything out of plastic, of course, what with the Arab oil embargo and all. It wasn't until the 1990's that Beltzig began reissuing limited editions of some of his crazier early designs--bookshelves, in-wall bench pods, a freaky Giger-looking chair called the Floris--but so far, nothing for the kids.
more pics: Made from Polyester: Play gear and kids furniture - Form Magazine 039, 1967 [form.de]
Three Beltzig children's chairs and a play table, 1966, price on request [Galerie Objekte Maurer]
Single red playtable [cominghome-interior.de]
POPDOM Collection Siekmann [sammler-paradies.com]
Froelichladen - Beltzig Sitzobjekte Auberge and Kinderkarussel 
Nov. 27, 2006: Lot 328 - Günter Beltzig. 'Auberge' seat-and table unit, 1971, est. EUR8-10,000 [liveauctioneers.com]
Beltzig Playdesign [beltzig-playdesign.de]