Way back in time, front mech were down pull, the cable running along the underside of the down tube. At some point, early 90s perhaps, mtbs appeared that had both shifter and rear brake cables either running along the side or along the top of top tube thus requiring a pull direction converter for the front mech. This usually was a small pulley wheel near the bb to reverse the cable direction. I still belive that gizmo in photo is doing the same and i guess it is there for originality even though it is not needed on that model of front mech.
I would keep it and fit period correct mech; its a bit special.