http://www.thehighroad.org/showpost.php?p=5281681&postcount=6
The current sporting style Express guns use a different system to retain the magazine spring and detent the magazine cap than older Expresses, and all Wingmasters, Police guns etc. That new system consists of a toothed plastic piece, retained in the end of the magazine tube by two pressed-in dimples, which both holds the magazine spring in place and keeps the magazine cap tight.
The older Express guns, plus the Wingmasters, Police guns etc., use a spring-loaded detent on the barrel ring to keep the magazine cap tight, and use a cup shaped spring steel magazine spring retainer which is press fitted into the end of the magazine tube to hold the magazine spring in place when the magazine cap is removed. This retainer can be pried out fairly easily if necessary to clean out the magazine tube.
The current production Home Defense version of the Express gun (with a factory installed magazine extension) has no magazine tube dimples (which interfere with the use of an aftermarket magazine extension on other newer Express guns), and has the old style spring loaded magazine cap detent in its barrel ring.
It's best IMHO to segregate barrels by type- old style versus new style, and stick to the design that is basic to the gun you have when shopping for spare barrels. Otherwise you have to use alternative ways to keep the magazine cap tight, like teflon tape on the threads of the magazine tube.
Keep in mind that the 21" vent rib barrels made for the Special Field version of the 870 will not interchange with other guns in the same gauge- the barrel ring is in a different place on the Special Field barrels. Otherwise all 12 ga. 870 barrels should interchange with no problem, save for the caveat about the two different systems for retaining the magazine cap and magazine spring described above.
lpl