It's not just the K6 & K8 sharing the same housing, the K10 too. It'd just made the identification much harder without looking at the burrs. But by far, the most commonly available second-hand K series is the K6 (90% of the time?). I can almost bet this is too. K6 wouldn't neccesarily be an upgrade to the MD40, which is a decent grinder itself. But the burrs may need replacing if you find its performance deteriorating. It shares the same burrs with Rocky which reportedly wear out sooner (compared to Super Jolly). The size of K6 is probably bigger than MD40 too. If you're looking for an upgrade, I don't think the K6 is the one (at least not a very significant upgrade). To look at the burrs, you need to unscrew the top adjustment collar as mentioned above. There's a grind locking nut (located normally at top,instead of side which is the hopper locking nut) that needs to be removed before you can do so. I noticed there're some variants to the K6 depending on the model years, so it may be a little different. Closer pictures would probably help. To adjust the grind, you need to unscrew (not press) the grind locking nut located at top adjustment collar