I recently needed to strip and reassemble another Original Model 50. Getting the 3 ball trigger unit back in place is an art in itself but I did find what I thought was an easier way.
I give here both methods: (numbers refer to earlier thread stages)
8. Re-assembly:
Method 1: Rifle in spring compressor. Fit mainspring and guide. Put the inner sleeve into the outer sleeve ... the inner sleeve needs the ‘join’ pointing towards the trigger slot and the outer sleeve has obvious cut outs for the trigger unit to fit. The outer sleeve fits with the open slot towards the end cap. Using the socket, compress the mainspring / sleeves until a small diameter nail passing through the rear spring retaining pin hole can be used to hold the tension. At this point refit the small spring and keeper to the top of the cylinder (under the scope rail). Carefully lever the inner sleeve down to gain space to fit the spring and push the keeper/spring
into location with a screwdriver.

Method 2: Fit mainspring and guide. Put the inner sleeve into the outer sleeve ... the inner sleeve needs the ‘join’ pointing towards the trigger slot and the outer sleeve has obvious cut outs for the trigger unit to fit. Assemble inner and outer sleeves with the small spring and keeper in place; the spring buts up against a flange in the outer sleeve while the ‘spring and keeper end’ slots into a slot in the inner sleeve with the keeper held in the end of the slot. The outer sleeve fits with the open slot towards the end cap.The spring is under some tension at this point. Hold unit together and fit over mainspring and spring guide. It is better to have the action with the scope rail upwards at this point. Compress the mainspring using the socket as above. The outer and inner sleeves will move inwards. Keep the small spring and keeper in place. When sufficiently into the cylinder, the small spring and keeper become captive.

9. Refit the proper pins to hold mainspring.

10. Refit trigger unit
Method 1: first fitting the pin. Then push the outer sleeve back to allow the trigger to fall forward before refitting the trigger spring. This can be most easily be done by using a small flat blade to push the outer sleeve rearwards until you can see the end of the outer sleeve down towards the mainspring slot. It is possible to insert a small flat screwdriver blade into the small gap to retain the outer sleeve against spring tension and so allow the trigger to fall into place, cocked. The trigger spring is best fitted by fitting the end over the ‘cylinder pip’ then compressing it with a large flat blade screwdriver until the end of the spring is flush with the trigger unit flange. Take care not to let the spring ‘ping’ away.Now slide the spring into place in the trigger unit.

Method 2: Hold spring in place on trigger unit and hold in location on the action. Push down on trigger/spring to compress spring and bring the holes in the trigger housing into line with those in the action. Insert a small drift through the holes to hold it in place. Then tap the pin through. It may be necessary to wiggle the housing to get the holes on the second side to line up ... but I found this method is less hassle and more ‘blood free’ than trying to compress the spring as in method 1.

11. Refit the anti-beartrap bracket (if fitted). Replace the end cap.

12. Put action back in stock and refit the underlever pivot bolt.

13. Refit the retaining ring on the underlever catch bolt.

Cheers, Phil