I really don’t think those fine lines that I showed in reply 8 were from any brush, even if they’d had access to synthetics.
It would be interesting to read the acid etching piece in your link flying free, but it only lets me view up to page 72, coincidently B for Brushes (says synthetic fibres only introduced in late 1960s, til then different types of animal hair). I agree with Paul, I don’t think any organic brush is going to last long enough to be useful in an acid aggressive enough to etch glass.
Another point against brushing is that I’ve read that when in the acid baths for etching (probably talking about needle etched stuff) the item had to be agitated to move the acid around and the progress checked. I think if the acid was brushed on there might be an inconsistent finish due to lack of control of quantities and timings.
Thanks for the new images.
I don’t think it was acid etched after cutting as they would have had to paint resist into the cuts and I think it looks too neat for that, in places the resist wouldn’t have kept to the line so precisely. That bleeding into the mitre in the middle photo could also come from an abrasive reaching into the cut. The bleeding indicates frosting after cutting, but to me, the neatness of the line indicates it wasn’t acid...bringing me back to an abrasive.
The finish does look more acid etched in these photos but where the lighting is better in the original photos it looks more like from abrasive. It might be down to lighting as it is very difficult to capture in a photo. If I rotate the middle new photo 90 degrees (so it’s the right way up), I think I can see a (now) horizontal direction to the finish...but only just.
A good place to look for clues is where the finish transits from frosted to gloss - but without a bordering cut - often where the bowl meets the foot.