I'm transcribing a book of 18th Century music and have come across several instances of the notation shown in the image. A few fellow musicians I've asked, and the online Dolmetsch guide to musical symbols, all draw a blank.
It's from a book of dance tunes 'for the violin or the German flute', and the marking is definitely printed in the original, e.g. not written in later by hand by the book's owner.
Edit: The image doesn't seem to be showing up for me at least. The symbol is directly above the note and looks like an equals = sign, only tilted at about 30 degrees (right end higher than left end). It's definitely a double line, not a tenuto.
Edit: Thanks for thoughts so far. Here's an image of the context. Note that a few bars later there's a tr trill marking!