Some mountain bikes with top-tube cable routing still used bottom-pull front derailleurs. They did this by mounting a little wheel, usually plastic, below the derailleur on the seat tube. The cable routed down through the wheel and back up to the derailleur. It's hard to tell with the kickstand in the way, but that little boss you're seeing may be a wheel mount. However, it looks like it would be too low and not have nearly enough clearance for a wheel with the chainstays so close. Perhaps it had some other sort of adapter that mounted there. Your best bet is to either try to make an adaptor yourself, or just buy a new derailleur. That's a great bike, by the way.