you're lucky - wish I could follow my oh around such venues - she thinks my purchases are a waste of money entirely, and wouldn't be seen dead with me at such places.
It's a very knowledgeable person indeed that can describe with such accuracy that you mention, the origin of pieces, and caution is a word that you learn in the world of glass - dealers and sellers are not always reliable - they have a vested interest to inflate attributions, and sadly it's a fact that people on occasions tell lies.
For obvious reasons, we are unable to comment on your glass without seeing pix, and even then it can be unreliable when making attributions from screen images only - this is a problem that afflicts clear drinking glasses especially, since C19 and C20 copies of earlier styles and fashions abound.
I'd suggest that it's the knop that is bladed, rather than the stem - the knop being the swelling in the centre (or thereabouts) of the stem. As you will know from your books, knops come in a variety of shapes and number, and in conjunction with other factors can be an aid to dating drinking glasses, sometimes.
The folded foot was originally a feature that the Italian glassmakers brought with them in the latter half of the C17, and can be seen on British pieces up until c. 1745/50 - it's purpose was to strengthen the outer edge of the foot rim. For reasons I'm unaware of it seems to re-emerge around 1820 for a decade or so and is found on glasses that also have a depression where the pontil scar has been removed.
As you'll know, the fold is made under the foot - but some mid C19 glasses have the fold over the foot - a feature of Continental glasses only, I think.
Older hand blown glasses - prior to c. 1860 - will always show either a scar or a depression where this has been ground away - the reason being that since the rim of the piece needs to be finished by the worker, this can only be done by attaching a pontil rod to the underside of the foot, thus providing the worker with a means of holding the hot glass. Generally, the older the piece of glass then the higher (more conical) will be the shape of the foot.
When the rim has been completed, the pontil rod needs to be broken away from the glass - hence the scar.
Subsequent to about 1860, a mechanical means of holding the glass - called a 'gadget' - was used instead of the pontil rod - thus obviating the need for a pontil rod and the scar it creates.
Generally speaking, the gadget leaves no perceptible mark on the foot of the glass - but what you will see on second half C19 drinking glasses is the 'Y' shaped mark - on the underside of the foot - where shears have been used to cut away the pontil blob in order to fit the gadget to the foot, thus leaving the 'Y' mark.
However, with British material, ground/polished depressions under the foot continued well into the C20 on better quality glasses, so this feature is unreliable completely in determining age. I've no experience of Continental glasses, which may well not share this time-line of characteristics.
With older hand blown pieces, it should be possible to detect the 'blip' on the rim where the worker cut the plastic glass away to create a symmetrical shape to the rim of the bowl - his start and stop position creates this noticeable thickening point on the rim.
Could be wrong, but don't think the Victorians produced glasses with your 'bump' in the centre of the foot - and as far as I' aware this feature is C20 and indicates a mechanized method of production (of the foot at least).
The answer to your last question is ........... there really is no specific reliable change-over date regarding features seen on the underside of the foot in drinking glasses.
The snapped pontil scar can bee seen at any time from the late C17 to the early C20
The ground/polished depression can be seen from about 1750 through to the mid C20
Pieces with the 'Y' shaped shears mark really only signify the second half of the C19.
You don't say which books you have - would be interesting to hear, as some are better than others, and would be interested to see one or two of your pieces. Charity shop glass, on occasions, can be genuinely old, but so often are copies of period pieces.
The above probably sounds confusing - and there's no short-cut to knowledge unfortunately - experience comes only with handling glass -
but at the end of the day, if you enjoy collecting that's all that matters.
Except of course that accuracy is much more satisfying than not knowing what you have.
As you can imagine we do have some really knowledgeable people here, so always possible that some of my comments might be corrected.
P.S. Welcome to the GMB by the way.