A front derailleur is actually quite a crude mechanism when you think about it. From the low end to the top end, or in this case, almost top end: Shimano Deore DX FD-M650 front derailleur, they all have worked almost the same.
The action is not unlike picturing your finger moving your chain side to side, it is actually possible to downshift with your foot if you do not have a front derailleur.
This is a Shimano Deore DX front derailleur in excellent mechanic shape, it has some scratches and some areas covered with black paint (which is easy to remove) since it was not removed while the bike it was attached to was repainted. However those issues are minor and can easely be taken care of (with some scotchbrite).
Down pull, clamp diameter is 28.6 mm, which for some of you is 1 1/8 inches.
Would look great on any hardcore vintage mountainbike.