Sylvania tubes

I have a bunch of Sylvania 12 type preamp tubes which are my favorite,the date code is printed on the green label ones but I have one yellow label one with no date code does anyone know when they produced the tubes with yellow label?
I use ( and like ) the Sylvania 12bh7a ( Yellow Print )Pinch Plates...i think they are from the Early 60's..I purchased them from ( ) Very nice tube....