It's a strange thing with these Supraphons that I've found... They seem to have kept making their vinyl well into the seventies of the same grade as the early to mid sixties vinyl of other labels.
While Decca, HMV, and Columbia started opting for a more... "modern" mix for their vinyl, of a more sort of creamy, flexible variety that advances in technology allowed (albeit no real noticeable drop in sound quality in my opinion), Supraphons such as this still have weight, thickness, and stiffness you'd associate with those earlier pressings from those labels.
Whether this was due to availability of newer kit and methods of pressing, or they just felt like sticking to the tried and trusted standards, I don't know, but they are very pleasing records to hold... They feel very substantial.