Join us on Facebook: Search for 'ConcoursMustang' Private group!https://www.facebook.com/groups/concoursmustang/Indiana SAAC Fall Classic , French Lick, IN Oct 11-12http://www.insaac.org/SAAC National-Sanctioned Concours Event
From the ones I have on record and pictures of and haven't looked into this in the past but gave it a quick look I see a mix of both year and month first in pre 66 stamped examples so far so it may be time related or as you suggested possibly two outside providers