I first thought it is standing for "Signature Series" but then I found pictures of the Original sersies having S/N starting with "SS", too. Like Fender uses "MX" for their Mexican crafted Fenders.
SS typically stands for Standard Series. If it's just a single S, that stands for a "Custom Shop" level production guitar made in the Sado Island facility. I believe that facility closed a few years ago.
Thanks! That answered my question :).
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