Invert Minis have become quite a popular marker since their release in 2007. They feature a very small body size and unique internal layout which makes maintenance very easy on certain parts of the marker which are otherwise more difficult to access. KEE/Empire has released many markers using the same firing assembly, with only minor differences to the design. Most markers use the same firing valve, bolt/spring, and solenoid.
The Mini design is based from a 2006 marker known as the King Cobra from the Taiwanese company JBS. Minis are very similar to the Cobra in some ways, although some of the marker's support features have obviously changed going into the Mini. As I understand it, the markers are both made by the same manufacturer in Asia, although Minis are owned by KEE Action Sports. The main differences are found in the new electronics/foregrip, the body design which moves the air chamber down to the grip frame, the firing assembly which has been reduced in size and complexity, and others.
Similarities between the markers include the general design of the firing assembly, regulator, electronics, and solenoid; all of which are extremely similar, so much that you can tell certain items were directly made by the same company. For example, the circuit boards both use battery-loss volatile memory which will reset its settings if the power source is removed. This was later remedied, along with several other quirks that were leftover from the King Cobra days.
Empire markers use a mixture of both metric and 1/16" AS-standard o-rings. Sometimes you can swap one for the other, but it often requires experimentation because the new o-ring will have a looser or tighter fit compared to the factory o-ring. For instance, the firing poppet's o-ring can be swapped with a 13/90 instead of the factory 2x10-mm ID o-ring.
Classic Minis and Axes use the same Relay ASA regulator. Information on regulator maintenance can be found here: Relay maintenance.