valve o ring size

hiya ive got my hands on a spare bottle. with a slight leak but its a different model valve. the 1st valve has a brass spring retainer nut and brass fitting at the top where the stem goes it., it has a black derlin valve bit for the stem and the stem is thicker.

the new valve ive got is all metal construction with a plastic spring retainer nut and white derlin valve but there is a recess inside for an o ring does any one know the size please as this was missing thanks