In the early 1950s, the earliest years of wider production of phenolic resin balls, both Les Usines and La Decalite used the "open 4" on their 4-balls and their 14-balls. I can not prove that your beautiful balls were made in Belgium, but I believe they were made in either Belgium or Germany. The Raschig plastics had a rather indescribable translucence in their more muted colors and the pictures you sent seemed to hint at a German made ball. It would be very difficult to prove that your balls were made in the U.S. at Albany. The number fonts are unique. Look at the 8-ball and see how small the top circle is compared to the bottom. Even SALUC in their more modern balls, never made their 8-balls that disproportionate. Keep in mind also, that the European companies were very good copycats. The original Hyatt Zig-Zag balls enjoyed significant popularity. Why not knock it off? One way to sap Hyatt was to sell their designs.
Thank you for sharing your set.