In 1943, a search of the newspaper archives gets 233 hits for "Milk Stout". For 1945, it's just one. Let's see if we can work out why.
This court case got me wondering:
"Berwick Petty Sessions
Before the Mayor (Councillor J. Fleming), J. W. Carmichael, A. Hay, Esq., and Miss Cockburn.
James Wallace, 5, Hill Crescent, East Ord, was charged with cycling without front rear lights on 2nd February.—Fined £l.
J. and R. Tennent, Ltd., Well Park Brewery. Glasgow, were charged with giving with a certain article food, to wit, milk stout, a label calculated to mislead as to the nature, substance and quality of the said article, the stout being sold by them to A.
Middlemas and Son, Ltd., Kelso, and thereafter to a local retailer.
Mr Stanley Strugnell prosecuted. The defendants were represented by Mr C. P. Forster, solicitor, Berwick, who pleaded guilty to the charge.
Mr Strugnell explained that Mr Arlidge, Sampling Officer, had purchased a bottle labelled milk stout, whereas the bottle contained only ordinary stout.
Mr Forster maintained that the article in question was not ordinary stout as it contained a certain proportion of lactose sugar, but not the quantity of lactose which had formed a part of pre-war milk stout. The defendants had been informed by the suppliers about a month after the date of the offence that the milk stout sugar with which they were now being supplied had not the same content as previously, and immediately took steps to have the milk stout label withdrawn.
Defendants were fined £5 and costs."
The Berwick Advertiser - Thursday 17 February 1944, page 3.
The gist, as I see it, isn't that Tennent's Milk Stout didn't contain milk, but that it didn't contain enough lactose. Which was the result of a change in the sugar supplied to them. It sounds as if it's purely about quantity of lactose used. Which is the same argument as in the case a couple of decades earlier.
"Slips that PassYou have to realise that during this period, when a Labour government was in power, ministries had lots of direct influence in various industries. Partly through nationalisation and partly through bureaucracy. ANd it seems that the Food Ministry, unlike magistrates before them, had decided lactose wasn't good enough to qualify as Milk Stout. Only milk itself would do.
ORIGINS WRITING in the tavernacular, a correspondent finds that there is no milk stout nowadays. Reason is that a year or so ago the word "milk" was dropped, as the Food Ministry did not like it and because no milk was used in its manufacture. It contains lactose, a form of sugar derived from whey, and was first produced in the early part of the century. Very many names in common use are misleading. Prussian Blue, popular artist's colour, orginated in this country and not Prussia. The Turkish bath, came from Russia and not Turkey, while Jordan almonds do not grow on the banks of the Jordan. The name is simply a corruption of the French "jardin," meaning garden. Brussels sprouts did not come from Brussels, and French beans did not originate in France. York ham (you've heard of it) did not come from York, it just got its name from "Y.C." a trade mark. Stilton cheese is made in Leicestershire and got its name by accident, just because the Leicestershire cheeses were picked up by the stage coach at Stilton. Irish stew Isn't Irish . . it came from Germany . . and lead pencils contain no lead . . only graphite. T. J."
Western Daily Press - Wednesday 25 May 1949, page 6.
Which leaves me wondering about the legality of the term today in reference to beer. Has that ruling been overturned in the meantime? Or is it just that no-one cares any more?