Dropped g's in upper-class 1930s Britain

‘Now take huntin'…’ ‘Oh, bull-fightin' — that's quite a different kettle of fish.…’ Italics bred italics. Dropped g's fell as thick as confetti.

(Jan Struther, Mrs Miniver, 1939; 4th chapter, “The Eve of the Shoot”)

The location is a shooting-party hosted by Lady Chervil. From that and other clues, the context is definitely upper-class British society. So the dropped g's are not a sign of a working-class accent. What do they mean?

They are a characteristic affectation of an upper-class (or would-be upper-class) accent from the first half of the 20th century, not much heard since 1950 or so. The terminal Ns are quite clearly enunciated. "Huntin', shootin' and fishin'" is a common(ish) phrase used to describe a certain category of people likely to attend country-house parties, who are pretty much the likely users.