Think this has crossed with Neil's, so hope he won't object if I've doubled up on anything.

I believe that bat is correct re c. 1800 -1820 for the revival of the folded foot.........think Peter also made that comment some time back, although such pieces aren't perhaps easy to verify - dating of drinking glasses from later in the C18 and on into the early C19 needs care......... partly not to confuse them with earlier patterns and partly to avoid modern copies.
The bladed/flattened knop in the middle of the stem seems to be one way of determining which century. Most periods in history are littered with the fashions and styles of earlier times, or at least the best of the bunch.
Ref. my quote ........... "Bickerton includes two examples with this sort of typical engraving, and he says late C18/1800" (from the Rudimentary Stem group)......... should have included the fact that neither of the two examples he shows has a folded foot. The fact that he included only two engraved dwarf ales from this
Rudimentary Stem group - plus the dates he gives - shows that he considered them to be late in the period of these ale glasses, and of a common form. The fact that he doesnt include an engraved piece with a folded foot
(within this group) may indicate that he didn't want to confuse later (Regency??) revival styles with features from earlier in the C18 (see below). That's just my opinion though.
Bickerton does, however, include another group - 'Plain Straight Stems' (drawn stem/bowl construction) where you will see genuine short/dwarf ales from c. 1740, which as you'd expect do have folded and domes feet, and which also have similar engraving of hops and barley.
So folded feet and barley/hops engraving occur at widely differing times and care is needed to remember which characteristics apply to which period so that you don't get the date wrong.
I don't know to what extent there has been C20 copying of these late C18 ales - as a type they seem not to excite the collector in the same way as do some of the high end wines etc.
Most of the books seem to suggest that alcoholic drinks from the C18 were very strong, and bowls mostly were correspondingly smallish, but suppose you could always get a refill.

Regarding the books - E. Barrington Haynes is a paperback and should be as cheap as chips - poor quality paper (not acid free so it goes that dirty shade of yellowey-brown) - as far as I know never issued as a hardback which was a shame.
Silber & Fleming is great even if only for browsing to see what the Victorians created for the home - amazing stuff.
Bickerton is a modern book and costs rather more - it's one of those Antique Collectors' Club volumes, but really worth having.
P.S. apologies this is long-winded, and if I have anything wrong please shout pronto.