I'm more or less guessing, but I would say it used to go where the vacuum modulator is now. I have heard of mechanical ones, but they were for really high performance engines that didn't make
any vacuum. That box sure looks like the same connection that a modulator uses.What is inside of it? At any rate if neither end is hooked to anything I would put it on that pile of stuff that "might be of use some day in the near of far distant future"