900x
Have a 900x in .177 cal and have removed my mod but was a bit of a battle..first prise out the plastic cap then remove internals at the bottom is a large nut 19mm I think you will need a suitable box spanner or thin walled socket with a long extension to remove but even then is hard to remove the mod I had to resort to sawing it off but I believe others have had more luck ..once removed you are left with am M12x1 thread and a step of different diameter 14mm I think ...
So could get a 12mm x1 mm adapter to 1/2 unf and fit any 1/2" mod but you would be left with the step
Hope this helps
Missed Again ! .... I Was On Him !!!!!