Grain whisky started to be distilled in Scotland around 1830 and since 1863 all blended whisky has been made from malt and grain whiskies. The difference between grain neutral and grain whisky is in the processing. Grain whisky can be made from any grain type but normally either maize or wheat to which must be added a minimum percentage of malted barley and, like malt whisky, no chemicals etc can be added to the production and it must be distilled at a specific strength. The wording states that it must be distilled to retain an identifiable flavour. Grain neutral on the other hand is usually made from whichever grain is the cheapest in the market [a lot of neutral spirit is made from starch] to which enzymes and additives can be added to speed up the conversion to alcohol. They're looking for near pure alcohol with no flavour that is normally **** to gin rectifiers and vodka producers for further processing.
As to flavour, remember that there was no distilling from 1940 through to 1948/49 so most blends in the 1950's were produced with much younger whiskies that they use now, in most cases only 3 or 4 years old whereas now most standard blends use whiskies up to 7 or 8 years old.