From my research Kester is a solder company. I would guess your assumptions are correct, but with one little correction in your math:
National lead's 666 alloy was 39% tin, 1% antimony, and 60% lead
But we knew what ya meant
Did you find any 111 markings on the 50/50 stamped bars. Supposedly that was National lead's marking for their 50/50 alloy (lead to tin)?