With everything cleaned and lubed, place the piston into the cylinder. You will then have to slide the cocking arm through the trigger housing until it drops into the cylinder and piston and then pull it towards the front on the gun. Locate the breech block in the jaws and replace the pivot screw and lock nut.
You can replace the mainspring, endcap and cylinder end pin now.
Locate the trigger in the housing and locate its pin. Place the sear spring in the recess in the sear and slide the sear into the housing and then compress the spring a little to enable it to go into the housing. Replace the pivot pin.
Put the action into the grip and replace the big screw through the grip.

My gun needs a new mainspring as the old one is very tired.
I have not plinked with the pistol yet, other than 2 shots when I got it. The rear sight is a bit like a BSA cadet type, that is not adjustable for elevation. This is different to the spring steel one in the gun that was featured in the March 2016 AGW.

I still find it odd that the break barrel and the underlever share no parts. Even the shoulder stocks are different in length, width and shape.