It indicates a chromatically raised sixth above the bass. Here are examples of the same measure from other editions, both of which use the more common (in my experience) slash through the figure. (Note that both are in E major rather that F major as in the OP.)
Both editions can be found on IMSLP.
I didn't find anything about that mark. I even checked Arnold's treatise (basically, thoroughly). Ignoring the mark still leaves a normal progression (C7 to F to B° following a cycle of fifths.) Obvious interpretations are, misprint, part of the next symbol, editorial interpolation,.... As mentioned in a comment, it would be good to see if this symbol is ...
It is not obvious what Bach's intentions were, though perhaps musicologists who have studied this may have a better idea. Most of Bach's works, including the Inventions, were not published during his lifetime, but circulated as hand copies until they were published in the 19th century. In some cases the earliest surviving copies (or different copies by ...
There are four primary sources for the Inventions and Sinfonias.
The Little Klavier Book for Wilhelm Friedemann (1722)
An autograph fair copy by Bach (1723)
Ornamented versions in a manuscript belonging to Bach's student Heinrich Nikolaus Gerber (1725)
Ornamented versions from an unknown student (ca. 1723)
Invention #1 is ...