The Ms260 and 390 are entirely different in construction. The 260 is a light durable pro saw very popular with firewood cutters, slashing crews, and pulp loggers. The 390, although somewhat more powerful, is designed as a semi professional saw. It will not have the service life of the 260 under hard use but is priced more for the farmer/rancher/serious weekend warrior who needs power.
The majority of Stihl products we see in North America are made in a state of the art factory in Virginia Beach. The MS441 and larger saws are built in Germany, the electrics are made in Switzerland. Stihl also has a huge factory in Brazil that built the grandaddy of modern saws, the 090.

Stihl has a plant in Virginia Beach, VA. Many US sold saws are made there. I do not know how Stihl divides which plants will produce which saws. I imagine it would be pretty simple to have saws assembled as close to point of sale as possible. I don't imagine there are many specialty items for assembly that are too expensive to have at all their plants.

What you see as a feature is to some a bad point about the 390. The 390 is very heavy for the power it puts out when compared to the 260 and other pro saws. That will be one of the biggest quantifiable differences between the lines. More power with less weight. After that, ability to service and repair along with durability come in but most consumers won't be concerned about that given reasonable durability. Some features are different such as oilers, decomp valves, anti-vibration and filtration. In the end it's the total package that makes the difference. If you aren't into chainsaws or small engines, the differences won't mean a whole lot to you. If you are only going to cut occasionally and just need something reliable, the 390 is a fine saw.

I don't think the Stihl ratings charts quantify the differences between the saws accurately. If you just look at power and weight, the difference is small. But when you run them side by side, there is a much bigger difference than those numbers tell.

The catalogue number indicates the country of origin, there is also a sticker (required by law) on the tool with the information.

canguy are you saying that Va Beach only produces certain models and likewise for other plants? Are the pro saws only made in Germany? I could see where logistically they would want to reduce the variety of parts each plant would need to stock, I'm just not sure what other logic besides logistics would play into that model. Something like "Stihl is German, therefore all flagship models will be produced in Germany".