Yes, it is. It is translated and transcribed in modern German (if, for example, your keyboard doesn't feature an ß) as "ss." It is called "s-z" or "scharfes s," though. Reading von Danzig, I've seen words spelled with "sz" where I'd write "ss." So I guess you're on the right track, even though I have no familiarity with those dialects outside of fechtbücher.
A correction concerning the "long-s" and the "round-s." I read up on this in the German
Wikipedia (not very scientific, I know). According to this, the long-s occurs only on the beginning and inside *words.* The round-s is used at the end of syllables and words. There may be no two round-s following each other, those have to be written "fs."
An example:
There is no way in written modern German to differentiate between "Wachstube - Wach-stube (guard room)" and "Wachstube - Wachs-tube /tube of wax)". With the old writing you get "Wachftube" and "Wachstube," respectively.
I hope that clears it up.
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