Whisky by the gill- make that two! And a tankard of ale with that
(partial tosspot and full-time snob).
The Bodleian has a song sheet, "A Tankard of Ale," but it doesn't display.
Harding B 25(1882), printed by Batchelar, London, bet. 1817-1828.
"Not yet drunk nor yet sober, but brother to both ..."
Does anyone have all the words?

It does not seem to be in the songbook, "A Tankard of Ale," comp. Theodore Maynard, c. 1919; online at Tankard of Ale

Tankards used by pubs to serve customers (pre-present decadent times) were stamped by inspectors, showing that they contained proper volume. The line on modern glasses is supposed to do the same, but no guarantees except the word of the server.

Not sure of British barbershop protocol, but pre-WW1, clients of American-Canadian barbers has their own exclusive shaving mug, and often badger brush, which the barber kept on a rack in his shop.

In pubs, there is only the server's word that the glass, used by many customers until it is broken, has been properly cleaned
between each use. Who knows, glue might have adhered! A personal tankard, on the other hand, is just that.