Daly's Bartenders' Encyclopedia by Tim Daly (1903, p.113)

0.67 glassale — old alebase
0.33 glasslager beer — One-third fill.other
  1. Two-thirds fill a large glass or pewter mug with old ale.
  2. Fill the remaining third with lager beer and serve.

The drink is defined entirely by the beer split.

See also: Shandy Gaff, 'Alf and 'Alf