If you notice below, there are 2 different DSO codes for the Buffalo NY, Pittsburg PA, Philadelphia PA, Cincinnati OH, Louisville KY and the
Washington DC.District Sales Offices for the 1966 model year.  While these may not be the only changes that Ford made to the DSO codes
during the model year, they do represent one situation where having the most accurate DSO code from your car have helped to determine
the change. 

While I am uncertain of when the code change actually occurred, I have been able to determine the approximate time when comparing the
DSO and DATE codes.  I imagine it was in the beginning of the 1966 calendar year (1-1-66) based on registry data.  However, to keep the
registry as accurate as possible, I have noted the latest and earliest dates in the registry's records in the DSO description field of the registry.

DSO information found in owner's manuals also help support a theory that the difference may have only been an address change as the
manual's First Printing (Aug. '65) has only DSO codes, which include the early ones for those mentioned above, while the Second (Dec. '65)
and Third (May '66) Printings have the address' listed.

A picture from the back cover of a Second Printing owner's manual is at the bottom of this page.