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Whilst it may not have been Blackett or his team doing the research, research in this area does seem to have been done(1), and there are books and papers written about it(2). 1- https://www.lesswrong.com/posts/kyZgEKzZZtJQTCSG2/examine-yo... 2 - https://people.ucsc.edu/~msmangel/Wald.pdf

The point is that it would not be a random place.

The complete absence of armor in the Mitsubishi Zero is often cited as one of the reasons why it became outclassed by American fighters later in WWII.

Also it's not like they had no clue which parts of an aircraft were important. The less-snappy but likely more accurate story is they looked closely at the parts without bullet holes in them to see what important components in those parts could use some more shielding.