You got me curious so I started looking on the web and there are a lot of ideas out there, but nothing definitive. The first theories were that it means "Santa Clause" -- told him where to enter. Others suggested that was better than "Soot" or "Smoke." LOL.
The most common suggestion is that old houses had tie-rods that ran front-to-back through the house often ending in a double-ess plate or a cast iron star-shaped plate that spread the load on the masonry wall. Maybe the reversed ess decoration was a derivative of those functional tie-rod ends. I found some chimneys tied off to the roof structure with a rod through the chimney and a decorative element to spread the load.
The most common suggestion was this was a standard trademark by masons on their work. But I sure can't find photos of other chimneys with a backwards ess on them. Some thought it was the initial of the mason who did the work.
Here's an anchor plate on a church in France (from Wikipedia):
As a reversed S, Ƨ was also used as a fractional Roman numeral, where it stood for the fraction 1/72.
According to symbols.com, the letter 's' was also used to denote saintliness or holiness and it was also use to denote 'spirits' (notably wine).
A "charge" strongly resembling a Ƨ appears in the civic coat of arms borne by the municipality of Haßloch in Rhineland-Palatinate, Germany. But that was quite a ways from their home in Germany, so I don't see a family connection there.
Some suggested this was a traditional decoration on Tudor houses, but I don't find any photos.
Great...now you've got me wondering about it and I'll probably never know.