I always thought the number on the back cover of the keyboard should match the sticker on the inside backplate where the keyboard itself is mounted to. Correct me if I'm wrong.
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Did he just swapped the internals of a 1386085 into a 1391401 housing?
No, he didn't.
1386085 is a p/n of an assembly of a back plate and an actuator panel.
some of my 1391401s do also have a back plate assembly with exactly same p/n as yours. Have not yet checked all of my Ms though.
Some of 1391406(UK model) might have a back plate numbered as 1386719.
According to one of my MCA mates, the number 1386719 is also written on the back label in small fonts along with 1391406.