The new mag will not fit the older model, it's now similar to a AA mag. Mod's on the older models screw onto the shroud. Changing the shroud guts out will make it better though. You would need a shroud to 1/2UNF adapter, barrels not threaded.
Benjamin Marauder .22 pistol, carbined- 18FPE
Benjamin Discovery .22 Multi-Shot, Hawke AirMax 4-12 X 40 AO/IR
CS 2400KT .22, BB 3-9 X32 AO/IR