POLARSTAR CO2 inserts are manufactured for both the UGS and CGS stocks and have models for both the 12g and 33g disposable CO2 cartridges in either variant available.
However, the inserts have more uses than just being paired with one of these stocks.
Since they are equipped with the same standard ASA threads that are on a HPA tank, they can also be threaded into a conventional regulator to make an extremely compact air rig.