Jack, I have a Remington Express as well. It comes from the factory shipped with a green stick plug inside the magazine spring. This is to limit the capacity to 2+1 for hunting. The gun itself holds 4+1 if you pull out the green plug.
Or maybe you are talking about the rectangular stakes/dimples stamped (?) into the end of the magazine tube. Remington decided on the later models that they would add a keyable safety and then changed the magazine tube design to take a spring retainer that turned to lock into the dimples to prevent the spring and spring retainer from flying out when you unscrewed the magazine cap. This design also was probably also to prevent attaching the magazine extenders, to be a little bit more politically correct.
I ended up modifying my tube and attached a Tac-Star extension anyway. It works great. Heres what I used and how I did it:
Drill (you'll need a hand drill and a drill press probably, though you can probably get by with just a hand drill)
7/64" drill bit
small round chainsaw file (will be about 6" long, 5/32" diameter)
220-grit or so fine sanding mandrel (1/2" diameter maybe? Just as long as it fits inside the tube)
Unscrew your cap, take out your spring, spring retainer, and follower. Remove your pump/action bars, and the bolt from your gun. Drill carefully through the dimples on both side (do one side, then flip it over and do the other side). Then clean up with the chainsaw file and remove whatever is left of the dimple. If you don't do a good job here, your shells will jam. Then lightly polish the inside with the sanding mandrel to remove the machining and filing burrs. After that, you can go ahead and use an extended magazine spring and screw on a magazine extender to replace your magazine cap.
Buzz, careful though, not all .22 LR magazines are exempt from 10 rounds if post-ban. Only fixed tube magazines. But I doubt anyone makes a high capacity law enforcement only post-ban .22LR magazine anyway, since they don't have very much military or LE value.