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This is just a guess: It could have been there was some worry that it would be miss-set in a shop near a transmitter then deployed to a fringe location, and not worked right there. Manual IF gain control would allow the user to get it right.
The post-war 630, and a few of it's later RCA made variants (but not all) lacked AGC, so clearly RCA had some reason to have an aversion to it for a while.
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