The spring cap and valve cap are both threaded on to the valve stem, to lock and avoid any damage from repeated impact loads to the thread, the two lock together as conventional "double nut" method, but further to this, the two have a taper arrangement. I would expect this to apply a firm load against the thread by clamping with an inward force. (Thank you to Delage expert Peter Jacobs for supplying drawings and information in many areas, allowing me to proceed with the very stubborn items with a little more confidence!)
Remains from little critter's food store..
The Delage project has been partly restoration and partly recovery. Many items had been completed a considerable amount of years previously but, have been stored in a damp environment.