TOPIC:

Modify a 14mm wrench as shown and the removal of the choke-Plunger retaining nut is easy (Unless some goof-ball used "thread-lock" ! Then some heat may be required. (Ask me how I know )
Grind down the surface to remove the inside chamfer ! This ensures 100% contact with the thin shoulder of the nut.

I got a bit stumped by this one as well recently, but the stumbled on the fact that, of the 2 or 3 14mm C-spanners I had, the slight variation in the thickness of the 'C' jaws meant one of them fitted on fine on all 3 chokes.

As Scotch says, although there's some initial stiction (maybe 39 years on mine but who knows) the brass-in-ally released fine with just moderate pressure and no damage to the brass hex flange.