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One of the oddities of UK brewing during WW II was the use of oats. In beers other than Oatmeal Stout, I mean. Though in those the quantity of oats was tiny - far too small to have any impact of the flavour of the beer.In 1943, the government, wanting divert some barley into food production, ordered brewers to replace 10% of their barley with oats. At most breweries this would mean a reduction or total elimination of flaked barley. The quantity of malt used, however, wouldn't change.
"Flaked Oats
In September, 1941, the use of flaked barley was introduced by the Ministry of Food in order to conserve malt supplies, and a large proportion of the brewing trade has achieved a substantial use of this material. It has now become imperative that there should be a substantial reduction in the use of barley, whether flaked or malted, while maintaining the present level of output of beer. Supplies of flaked oats will be available from the present suppliers of flaked barley, and in most districts these supplies are already sufficient. A proportion of the barley crop is required for use in the loaf, and it has become necessary in the national interest to ask every brewer to take steps without delay to obtain delivery of flaked oats and to carry out experiments in order to ascertain the maximum proportion of this material that he can use. It is hoped that brewers will lose no time in putting the result of their experiments into effect. In some cases breweries are working to the full capacity of their mash tuns, and the greater bulk of oats, necessary to replace a given proportion of barley or malt, presents a difficulty. It may be possible to overcome this difficulty by using ground oats in place of flakes, and there is no reason why a brewer who can more conveniently use oats in this form and has the necessary facilities for grinding should not take his supplies in grain instead of in flakes. It should be said, however, that the malting of oats will not be permitted, as this would be uneconomic in malting labour having regard to the lower output of beer obtainable as compared with that from a corresponding quantity of barley malt."
The Brewing Trade Review, March 1943, page 63.
The yield from oats is significantly lower than that from barley Which you need more of it to produce a wort of the same gravity. Which could be a problem if you were already brewing with a full mash tun.
You can clearly see that in Whitbread's recipes from 1942 and 1943:

Whitbread beers in 1942 - 1943
Year Beer Style OG pale malt choc. Malt crystal malt mild malt PA malt wheat malt flaked barley flaked oat
1942 XX Mild 1029.1 12.02% 68.11% 2.00% 12.02%
1942 IPA IPA 1032.4 53.50% 5.73% 27.71% 6.69%
1942 PA Pale Ale 1040.0 3.30% 76.65% 9.07%
1942 XXXX Strong Ale 1044.4 2.80% 79.44% 9.35%
1943 XX Mild 1028.3 46.39% 10.31% 18.56% 17.53%
1943 IPA IPA 1031.4 6.52% 82.61% 10.87%
1943 PA Pale Ale 1039.2 4.97% 72.93% 13.26%
1943 XXXX Strong Ale 1043.8 2.86% 71.43% 17.14%
Whitbread brewing records held at the London Metropolitan Archives, document numbers LMA/4453/D/01/109 and LMA/4453/D/01/110.
The quantity of oats was about 50% higher than that of flaked barley.
Using oats was a very temporary occurrence - 1943 only. The next year everyone reverted to using flaked barley. What it does do, is throw up some odd recipes for people like me.
Given how high the percentage of oats was - 17% in the case of Mild and Burton - there must have been some impact on the character of the beer. Surely some drinkers must have noticed?