This one is an anomaly of sorts. Generally speaking, this Westinghouse Ivorite material came into use after the shop changed to the inclusive “S” stamp. I have seen a few 7 spacer knives with this material, a few. More examples showed up during  the three thick two thin spacer arrangement period, but it seemed to come into its own in the thick center spacer period.  I think that this knife could have had 7 spacers if the collar was not chosen as an option.  

 

So the quagmire is the separate “S” stamp and the handle material together. I can only surmise this blade had been sitting around for a few years prior to assembly, thus making it pretty uncommon.

An outstanding example for the collector.