"As fermentation nears completion, the yeast settles in a compact mass on the bottom of the vessel, the supernatant beer gradually clearing itself, its condition for running off to the store casks being determined by its "break," gravity, and temperature, the former being judged by the brilliancy or otherwise of the liquid, which has settled for an hour or so after filling, as compared with a sample drawn fresh from the tun. If the malt was of good quality, the brewhouse manipulation conducted properly, and the yeast pure and strong, the first specimen will have dropped quite bright, the yeast having settled firmly on the bottom of the glass, while the sample fresh from the tun will exhibit a perfect "break," i.e., the yeast cells floating separately in a clear beer. Another factor determining the casking of the beer, is whether it is to be stored for a short or long time; if the latter, then it will be run down at a higher gravity than if intended for a shorter lagering, as is the case with a quick draught beer. Before dropping, the scum or thin head is removed, the beer is then drawn off through a special adjustable pipe, which is pushed up from underneath to just above the yeast level, and, when all the beer hits ran out of the vessel, the yeast is removed in three portions, the two first layers through a bung-hole situated in the side, and the residue through an outlet in the bottom of the vessel. Only the middle layer is reserved for pitching purposes, the other two portions being removed to the yeast drying department, whence it issues as a valuable tonic food, the sale price of which is about 2s. 6d. per pound, rather different to the price obtained by English brewers, who, if they do not wash their yeast away, have to sell it for as much per hundred weight. The pitching yeast is kept under pure iced water until required for use."
Journal of the Institute of Brewing, vol. XVII, 1920, page 496.
That's a very different method of yeast collection compared to a standard UK brewery of the time. The initial dirty head was skimmed off and when there was a clean, white head, yeast was removed using a parachute. That's a cone that was dragged dalong the surface of the fermenting wort.
I'm scratching my head as to why Lager yeast was more valuable as a food product than top-fermenting yeast. Perhaps it's because it was cleaner, being from a pure yeast culture, unlike the mixed strains usually employed in UK breweries.
At Barclay Perkins in 1925, primary fermentation of their Lagers uusally took two weeks. They seem to have employed a pretty traditional cold fermentation, pitching the yeast at 48º F.