Because I'm pretty sure that no-one send out beer unfined any more. In the interwar period brewers increasingly fined beer before it was sent out. For one simple reason: they didn't trust publicans not to cock the procedure up.
"London and Burton beers require finings, which have the effect of clarifying the beer. Finings are sent with each delivery of beer, as a rule, and the amount to be used varies with the beers of different Brewers. Some need half a pint, some a quart, per barrel.Why did specifically London and Burton beers require finings? It's odd that beer from two specific towns should be singled out.
Some beers, especially in the country, are delivered already fined.
All beers delivered in cask require some hours' rest, preferably at least twenty-four (except London beers, which require less than most), and others longer, according to the class of beer, the temperature, length of journey, the ventilation, and peculiarities of the cellar. The time will be gauged by experience, and from specific information given by your Brewers. The casks must be placed on the scantling, and propped with wedges with the bung-hole or vent uppermost, and so that the cask is at rest, and, if anything, tilted slightly forward, not backward, as many publicans do.
During the rest, the sediment and the hops gradually sink to the bottom of the barrels.
They condition and flavour the beer, and some beers improve in quality, for a time."
"The Art and Practice of Innkeeping" by Alexander Francis Part, published by Heinemann London, 1922, page 199.
I'd have thought 24 hours to settle after delivery was the bare minimum. A couple of days was probably prefereable. I've another question: why did London beer require less time to settle? I honestly can't think of a good reason.
Interesting that the author seems to assume that all cask beer was dry hopped. While that's true of Bitter, it didn't necessarily apply to all types of beer, such as Mild. Barclay Perkins, for example, dry hopped their Pale Ales, Burton Ales and most Stouts, but not their Mild Ales. Though the bottling version of XLK, their Ordinary Bitter, wasn't dry hopped.