Mick,
The air slide has 2 narrow slots cut across it near the top - the portion between them is punched in to locate in the reduced diameter near the bottom of the plunger - thus preventing it from being removed again. With the passing of time, the plunger and the bore of the slide become worn and eventually the slide drops off - as has happened on your machine. Possibly, only Douglas fitted this type of choke assembly on Amal 274 carbs, so it is unlikely that anyone is making repro parts for them. Personally, I have never found these choke assemblies very effective, or easy to use. The Mark series engine rarely needs the choke - I have always found that a cold engine starts easily if the carbs are 'tickled' first - with the engine getting warm enough to run smoothly as the fuel level drops back to normal. All my engines are now fitted with carb tops intended to be used with cable operated chokes - with threaded plugs filling the holes for the cables - no more problems with self dismantling chokes, or weather getting in around the poor fitting assemblies.
Regards,
Eddie.