Get ready to put it back together. The piston was solid but crude It looks like it was made of tube, with the end bit put in and spot welded and then ground off. The piston rod appears to be brazed in and there was some hard crud round it (old flux?).
Assuming everything is lubed and you have the piston washer back on the piston, replace the piston, mainspring and guide in the cylinder( I used the old mainspring, which was a bit tired). Get the trigger housing bit located and then push down on the cylinder and locate the pin. Get the trigger into the cylinder and line up the pin hole with a punch a "slave" and then push the pin through carefully to keep everything straight. Locate the trigger spring on the pip at the back of the trigger and compress the spring and use a small screwdriver to locate it on the tang on the trigger housing. You can now replace the trigger adjusting screw and endcap, (remembering to line up the screw with the hole in the cap).
Get your barrel, breech block and cocking arm and feed the cocking arm through the bracket on the cylinder and replace that pin. Replace barrel pivot screw and lock screw. Drop action in stock and replace the front and rear sock bolts. If the holes dont line up, you have probably not tapped the cylinder endcap on far enough.
I reckon this is a crude little gun. The piston is rough as I mentioned and the back of mine seemed a bit oval. The breech of the gun has 2 spot welds holding it to the cylinder which look ugly when you see them. The trigger is like all the other small guns mentioned but is adjustable. I think this was an early version as there was no scope rail and the trigger housing seemed to be in one piece.
not seen an Original 22 yet but this little Bsf is better made than the other guns I mentioned, including the Hw25 ( which I think is very overated). Even with the tired spring I left in it, it is fairly punchy and great fun.