Food for thought, but I don't wanna overthink this.
Here's my take: if the Feds consider a serialized receiver a firearm, and if they _really, really_ cared about keeping import marks on firearms sacrosanct, they would stipulate that they had to appear on the receiver, not a replaceable component. Also, assuming the receiver is in some re-import or Military Assistance Program database, then they have the means to track it back to Korea if they need to. If not, it's currently in the People's Republic of California's illegal DROS database as produced by Springfield Arsenal and owned by me, with no reference to Blue Sky or Korea.
As for the Blue Sky quality thing, I agree in my case. My own rifle was pretty nice except for the stock (foreign replacement, kindling condition) and a couple of the springs, which I've replaced. My barrel has a bit more throat erosion than I would like, but another owner might not care. Bruce Canfield said Blue Sky guns were mostly beaters that the Koreans no longer wanted in his huge book on the Garand (which I just got for Christmas!) so what do we know.
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The replies here have drifted off topic -- could we please return to the original question, namely recommended M1 Garand gunsmiths that one can ship work to?