Middle of rearward side of drum has a small knurled wheel with a pin in the centre. Push down the spring loaded pin with a small tool, and while it is down rotate the wheel 90 degrees counterclockwise. The wheel can now be pulled off.
Gently pull on the peripheral edges of the cover, and the whole thing will come off.
Now you can see the guts inside. This consists of a central spring and several 'propellar' arms coming from the center. This system is under spring tension. rotate the propellar arms clockwise about 1/8 turn. You can now remove the 'dummy' cartridge chain from the loading port area - this is pulled into the drum of the mag and then out. Be careful to hold onto the spring loaded propellar assembly.
Now release the propeller assembly very gently, counting the number of turns until the spring is completely relaxed (eg 2 1/4 turns)
Now you can remove the propellar and spring assembly, and the unit is a condition to enable easy cleaning.
When assembling be sure to 'wind' the spring the correct number of turns (and partial turns) and insert the dummy cartridge chain to keep the thing in place.
Should work out OK, but no guarantees. Bear in mind that these are designed to enable relatively lowly educated troops to strip and clean them in the field, so a little common sense and patience is all that is required.