We've now moved post-WW II, but are still looking at London Ordinary Bitter. Which, as the 1950s turned in the 1960s was sidling its way past Mild to become the most popular type of beer.
The set of beers I'm basing this definition on were remarkably consistent, the weakest being 1036.3º and the strongest 1037.9º. They average out to 1037.1º, which, coincidentally, was about exactly the same as overall average gravity.
The grist mostly consisted of four elements: base malt, crystal malt, sugar - usually in the form of No.1 or No. 2 invert - and flaked barley or maize. Though the former mostly disappeared in the 1950s when brewers were no longer compelled to use it. Sometime the base malt included some mild malt in addition to pale malt.
With plenty of UK hops available - enough to cover the needs of the country's brewers - few hops were imported. Those that were used in Bitter were mostly classy Central European varieties like Saaz and Hallertau.
|Postwar London Ordinary Bitter|
|SRM||5 - 9|
|flaked barley or maize||10-20%|