Mine seems to already have the block in it, since it only takes two shells in the tube.
I haven't quite figured out how to get it out 
From AR15.com...
There is a pin at the top of the magazine tube called the magazine locking pin.
Push this down and to the right and pull it out until it stops.
Using it as a lever, rotate the magazine tube counter-clockwise about 1/4 turn until it stops.
Pull the magazine tube forward until it stops.
Move the action slide/forend assembly forward until it stops. There is a action slide handle retaining spring that will keep the forend from falling back, it is a flat piece of steel. Rotate the barrel and magazine assembly 1/4 turn counter-clockwise and pull it away from the receiver.
There is a round wire spring inside a slot in the magazine locking pin. Press this spring into its slot to clear its fat rolled end and pull the pin out of the magazine tube.
Remove the 2 opposing screws at the front of the magazine tube. Remove the magazine plug and stop plate toward the front. Be careful, the magazine spring is pressing on this and it could get away. There should be a wooden plug just under this, remove it and reverse the above directions for reassembly.
You will marvel at the machine work and hand fitting that was used on this fine shotgun, all the moreso that is was a production piece, meant for mass production. Only the finest materials were used to manufacture it, nickel steel early on then Winchester Proof Steel- Chrome-Molybdenum Alloy Steel later.
