Reprints from the US World Around Us series (often with modified covers) as well as new issues (#526, #528-#534) from the joint European series.
Issues do not have publications dates or frequency listed. Based on back cover coupons, they were issued in batches (e.g., #516 lists #501-516 but #517 lists #501-524 suggesting that #517-524 [8 issues] were published at one time).