A short explanation of STS Abel of Zagreb Glass:
In 1921 four glass factories in Jugoslavia merged to form a new company called Sjedinjene Tvornice Stakla N.D. (STS) which translates into "United Glass Factories", with its head office in Zagreb. Those four factories were Hrastnik, Rogatec, Rogaska Slatina, and Daruvar, each located in a different town, the first three in present-day Slovenia, and Daruvar in Croatia. Their STS catalogue for 1936 shows the trademarks below on the left. I believe there is another trademark of a stemmed goblet inside a star which was embossed on some of their glass.
There are very few publications in English about Jugoslavian glass, but one good article is "Glass in the Celje and Kozjansko Regions from the Seventeenth Century until Today" written by Jože Rataj of the Celje Regional Museum in Slovenia.
STS ceased to exist after the second world war when all the glassworks in Jugoslavia were nationalised under Marshall Tito's communist regime. They re-emerged as separate companies in the 1990s. One of the four original STS companies was Rogaska Slatina, a glassworks with a proud history which they date back to 1665 and which still today makes a wide range of high quality cut crystal and art glass under the name Steklarska Rogaska Slatina. The Classic Crystal retail website shows Rogaska products and their history listed in company with the best crystal companies world wide (Waterford; Lalique; Baccarat; Kosta Boda) - Crystal Classics, Rogaska Crystal.
The other one of the four still thriving today is the Hrastnik glassworks which was built in 1860, but was preceded by a glassworks called Jurkloster operating in Hrastnik since 1790. Today Steklarna Hrastnik have a €27m recently-built factory making glass containers and two other factories making lighting products and general glassware. Information about the company is on their website at Hrastnik 1860..
Daruvar, where the third STS glassworks was located, is famous for the Daruvar Cage Cup, a rare Roman glass vessel unearthed at Daruvar in 1785. Today Daruvar has a factory in Lipik making automotive, flat, and industrial glass products.
These are some of the glassworks with their massive new factories that some British and USA glassworks have been using to outsource glass production in recent years.