VERIFIED SOLUTION i

How to understand why PRF's Dec 2016, Jan 2017 and Feb 2017 are making all ZIP Codes starting with 055 invalid in MailStream Plus

MailStream Plus, all platforms
Whether MailStream Plus (MSP) position 28 ZIP IN was checking at the 3- or 5-digit level, anything that started with 055 was invalid. 

The reason was that starting with the Dec 2016 PRFs, the USPS removed 055 from their data.  That was because the only ZIP Code in it was 05501, and that was assigned to the Internal Revenue Service for tax filings being sent to them.  The IRS said they were no longer using that ZIP Code, so the USPS removed it.
  
But later it was discovered that 055 was also used relating to some 'Label To' destinations, and so problems were occuring.  The USPS added it back as a valid 3-digit with the March PRFs.

(Although no commercial mailer should be sending mail to it, at least one MSP customer has a client who was sending advertising mail to some 05501 addresses, and MSP was rejecting the mail.  So the March PRFs resolved his issue too.)
UPDATED:  August 14, 2017