The Shimano 7400 series derailleur went through, as far as I know, 3 revisions. The original was the 7400, 6 speeds.
Next was the 7401. Labeled for 6 or 7 speeds (in fact, the whole series will work up through 8 just fine). The cage has changed where the tension pulley attaches -- it has straightened.
Finally, the 7402. This was the 8 speed model. Similar cage to the 7401. The cage pivot, however, has changed, and is no longer exposed when the derailleur is mounted. It instead is accessed from the backside of the derailleur. The return spring is also more easily tuned, and a block to prevent the chain from jumping out of the derailleur has been added near the tension wheel. Prior models only had them near the jockey wheel. The marble hued name plate is also pretty snazzy.
Interesting note -- the cable anchor bolt on every one of the above pictures is not installed the way Shimano intended (or, at least, not on the 7402 and 7401. Its just an assumption with the 7400). The bolt portion should, in fact, be on the inside of the parallelogram, and the outside should instead merely have a flat, circular plate to hold the cable on. This is a little "cleaner" and perhaps more visually appealing -- it is, however, a pain in the neck to adjust when the derailleur is mounted, as the allen key must be maneuvered inside, while holding the cable tight. They work just as well as seen in the pictures, and are a whole lot easier to adjust IMO.
