It seems like the current procedure for checking serial numbers has changed from what is written in the guide, including the change of the apple serial number check url from:
https://selfsolve.apple.com/
to
https://checkcoverage.apple.com/
and the suggestion (from
An iDiot's Guide To iMessage ) to use the everymac url tool below to validate, and THEN check for existing serial registration with Apple.
http://www.everymac.com/ultimate-mac-lookup/
Furthermore, it seems like the current ideal response from Apple (according to iDiot's Guide) is "We're sorry, but this serial number is not valid. Please check your information and try again" - which is not how the current chapter 4.1 is written. Since this is all confusing as hell, could you possibly update it to reflect the current reality? This guide is such an amazing resource, but these small inconsistencies degrade the credibility and are cause undue frustration.
Many thanks for all the work in putting this together!