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The marks are Italian and this type of mark was introduced in 1968 which means your set isn't terribly old but it is one of the nicest I've ever seen. The BO stands for Bologna which is the city of assay and the number is the registration number belonging to the maker. In this case it is Argenteria Stefani. For more details see here:
Just to fine tune the earliest possible date for the use of the 'Star' mark, although the law introducing the new marks was enacted in 1968, it was not passed until December 1970, and allowing for the manufacture and distribution of the new punches, it was probably well into 1971 before it was possible that the new marks made their first appearance.