Milwaukee/AEG has a history of supplying motors to other manufacturers. Some of their motors are used by German tool manufacturers who don't manufacture their own motors. Rothenberger, the German pipe tool manufacturer, used Milwaukee cordless batteries for some tool models. Fromm, a company that makes steel strapping tools for packaging also has used cordless Milwaukee/AEG for some of their tools.
Unfortunately, some of the companies who used motors or battery systems from other companies have a habit of switching from one manufacturer to another, so the battery systems don't always stay consistent.