These are mono broadcasts from the 1960s. The quartet recorded Beethoven's Op 18.1 for Supraphon in the same period, in stereo, coupled with Op 95, and would soon make its first stereo recording of Smetana's e minor quartet. I haven't previously come across any commercial recording by them of Mozart's K499, though in the mid 1950s they made a marvellous mono LP of K421 and K428, which is now available on Testament, coupled with K 464.
The source tapes are good mono, and don't have the gloss of the 60s Supraphon sound, which made the quartet sound a little bass-light and sometimes almost aggressively accurate - which resulted among other things in two very determined and spectacular discs including the Beethoven Grosse Fuge which seemed to leave nothing to chance. These present a much more rounded and open-minded speculative, exploratory style of playing, in which the quartet is concerned with what the music might have to say, unfolding rather than packaging it. The contrasts of tone-colour are fascinating. Their accuracy is in no way diminished, and their phrasing, in the Beethoven particularly, is a joy. In the Mozart - the most Jane Austen-like of his quartets, the uncertainties with which the music flirts aren't underplayed, and in the Smetana they give a perfectly judged and very human performance of a work they were fully capable of playing in their sleep. They were probably the greatest of the European quartets of their day, and this disc shows them at something like their best.