http://www.mabuchi-motor.co.jp/en_US/technic/index.html
Here’s a reference guide that some may find interesting, regarding DC brush motors in general (and Mabuchi’s product line in particular).
They’ve got animations and other pages which explain the theory behind motors. It also offers a lot of insight into the reasons why motors that look the same on the outside can be very different on the inside.
Mabuchi is a top-notch motor manufacturer, and their production capacity is very high – I recall on a tour several years ago hearing that they were making more than 5 million motors per day. Clearly, production at that scale must be highly automated. Just as clearly, even relatively rare instances of manufacturing defects will be noticed quickly at that scale. Most of Mabuchi’s customers have a VERY low tolerance for motor defects.
The reference material linked above is well worth the time you will spend reviewing it.