This is for a pre war, full stocked Diana 25. This is the one with the screw on end cap. I've been told earlier ones had the cap inside the cylinder.
The strip is similar to the post war model and the British made ones, with only a few differences.
Undo the front stock screws and the rear one that is in front of the trigger guard and lift the action out of the stock.
Undo the cocking arm pivot lock screw and then the pivot and remove the cocking arm.
Undo the barrel pivot lock screw and then the barrel pivot screw and remove the barrel. It looks like there should be shims on the breech block but they were missing on mine. If you want to remove the barrel plunger, depress the plunger and then undo the grub screw on the LHS of the breech block. Release the pressure on the plunger and it and its spring should come out. You can clean,lube and replace it now.
Undo the cylinder end cap.
Remove the trigger spring from the tang one the cylinder sleeve. Undo the lock nut on the cylinder/trigger pivot screw. Put some pressure onto the cylinder sleeve and the pivot screw should push out. Remove the trigger. Release the pressure and the sleeve should come out and then you can remove the guide,mainspring and piston.
The piston has a leather cup washer and a fibre spacer. They are held on with a screw. If you need to remove or change these, you will have to drill out a small pin on the side of the piston, that is a keeper for the screw. Luckily mine was in good condition.
Clean and lube everything and go to Pt 2