"Those are Cinderellas & not real stamps or proofs. After the Six Day War dealers here in Israel made tons of material for collectors because it was a very popular theme among the collectors. Old dealers told me it was possible to make a fortune on material of this kind"
This is along the lines of my original thoughts. They are not valid for postage! I am guessing that the STS is a dealers, artist or printers logo/initials.
I don't think this is a KKL/JNF item as these labels were produced for charitable purposes and have KKL on them. It could be the STS is trying to imitate KKL. I have checked several old catalogues and they are not listed. Several Israel dealers put their logos or initials on material especially things like maximum cards.