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 The CO2 stem has a sharp pin on the front end that pierces the 12 gram CO2 cartridge on the first shot. It's original to all 1740, 2240, 2250B, 1760, 2260 and Crosman Custom Shop CO2 models.

 The large PCP stem fits both Discovery models and the Challenger. It is commonly used for bulk-fill, HPA and PCP conversions on 2240, 2250, 2260, Crosman Custom Shop and similar models.

 The small PCP stem is for the Marauder rifles and pistols, 1701P and 1720T. It also makes an excellent upgrade for the Maximus which originally has a 13XX stem. 

 The 13XX stem is the current version used for all modern 1377, 1322, 2289G Backpacker, PC77 and 1300KT models. It is a direct replacement for all older models as well. It is the same stem used in the Maximus, 760, M4-177 and many other models.