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I have exactly the same issue as @Johnbon jovi described on my MBP Retina 13 late-2013. Serial number "Unavailable" Blank Board Serializer v1.1 does not boot off the USB on this MBP.
The battery is now connected but the serial number is not back.
Please do not suggest take it to Apple as it’s end-of-life and out of service contract.
Was this issue ever solved ?
If so how ?