Very interesting thank you for sharing.
This is Chess Bazaar's current wording: "In the 1950s and 1960s the Zagreb chess set was one of the most popular chess sets used at the major international chess tournaments. It gained popularity when it was used in the World Championship Candidates Match that was held in Bied, Zagreb in 1959."
This is how misinformation spreads. Someone make an inaccurate or false statement, and then someone else repeats it without verifying it and sometimes alters or embellishes it, and then there are more and more repetitions and changes, with the earlier iterations being used as citations for the later ones, etc., etc..
I bought the Zagreb '59 set from Frank when he first released it. I was not that into the history of the sets at the time and bought it on Frank's recommendation.
I just did a Wayback Machine search and here is Frank's initial blurb on the set which is on his original House of Staunton webpage:
From 6 September to 31 October of 1959, eight of the world’s strongest Grandmasters competed in a powerful quadruple Round-Robin Tournament, played in Bled, Zagreb and Belgrade, Yugoslavia, to determine the challenger for the crown of World Champion currently held by Soviet Grandmaster Michael Botvinnik, Competing were Pal Benkö, born July 15, 1928, in Amiens, France; the young American Robert James Fischer, born March 9, 1943, in Chicago, Illinois; Grandmaster Svetogar Gligoric, born February 2, 1923, in Belgrade, Yugoslavia; Paul Keres, born January 7, 1916 in Estonia; Fridrik Olafsson, born January 26, 1935 in Reykjavik, Iceland; Tigran Petrosian, Born November 17, 1929 in Armenia; Vassily Smyslov, born March 24, 1921 In Moscow, USSR; and Michail Tahl, born November 9, 1936 in Riga. The tournament was ultimately won by Michail Tahl who went on to successfully capture the world title from Botvinnik. Fischer tied for 5th place, but had the satisfaction of wining 2½ out of 4 against Keres. To commemorate that classic struggle, The House of Staunton, has recreated the Zagreb '59 Series, Staunton pattern chessmen. These stunning chess pieces are accurate reproductions of one of the most popular chess sets used in the major International tournaments during the 1950s and 1960s. We have taken the liberty of making some minor modifications, such as replacing the ball finial atop the King with a more traditional Formeé cross to conform to modern Western tournament praxis. These chessmen could have the Kings’, Queens’ and Bishops’ finials with reversed colors, or only the Bishop with the reversed colors. The King stands 3.875" tall with a 1.75" diameter base. The Zagreb59 is robust, heavily weighted (47 oz.!), and is available in a Natural and Ebonized boxwood finish or in Natural boxwood and Rosewood. This set is designed to withstand the rigors of practical play while maintaining an elegance which has become the hallmark of a House of Staunton chess set. The design, quality, workmanship and finish of each piece is superb and the detail on the Knights is truly impressive for a set in this price range. The chessmen are cushioned in green billiard cloth and housed in a fine, color coordinating leather case, with our logo embossed in gold on the lid.
What I did not and still do not like about that set is the base diameters of the pieces are too small for the piece size.
After learning of the real set used for those tournaments, I find it kind of humorous that this created Zagreb design has the opposite base dimensions of the original set used. The Dubrovnik set has really bulky and great base sizes.
I don't know what HoS's original description of their "Zagreb" chess sets was. Currently they say, "The design of the Zagreb chess set is an adaptation of the most popular Chess set used at major international Chess tournaments throughout the 1950s and 1960s, most notably the World Championship Candidates Match that was held in Bied (sic), Zagreb in 1959." Note that they refer to it as an "adaption", which while not actually false, is misleading. However, they also refer to it as being an "adaption" of "the most popular chess set used ... throughout the 1950s and 1960s ... most notably ... Candidates ... 1959", whereas, actually the Dubrovnik (and never any "Zagreb") design sets were only ever used in events in Yugoslavia, per the existing evidence.