NOT easy.

I tried, a few times, it may be wording, but "legal" and "statutory" yield almost identical replies; and "nobody" is prepared to tell me when marking pints became statutory.
However I'm fairly certain that the left hand one is a fair old age, from its appearance.
Investigation proceeded as follows:-
Left hand one - 368gm.
Right hand - - -  305gm.
Left hand - A measured pint filled it to the rim and you'd barely be able to lift it without spilling, courtesy of a positive meniscus.
Right hand - When poured skilfully/carefully, it stopped about 1/4" (6.35mm) short of the lip, allowing a pint + a decent head.
It seems the humble milk bottle also suffered a significant weight loss; which will eventually, also, be illustrated.

I'm not sure if I'm a Glassaholic, or a Glasstomainiac; but one of these days I'll illustrate why I say that.