Hello,
REGDON = Registered on (Regd on)
735122 is likely to be the registration mark/number. It is not in the Glass Association 'Blue Book' on the subject.
I agree with Frank that the glass appears to be mould blown. You don't mention a silver mark, so I assume that is just referring to the colour, since this sort of glass is unlikely to have a solid silver stopper.
Nigel