Serial number (last 6 digits of VIN) apply to all series of 1948 and later trucks, 1961 and later Econolines, 1966 and later Bronco's.
6 digit DSO: First 2 decode to specific Ford District Sales Office that the dealer ordered the vehicle from.
Last 6 is the Domestic Special Order number. 1277 is related to your Econoline, but 1276 and 1278 could be any truck, Bronco, Econoline or a Passenger Car.
1972 serial number range: M40,001-Q00,000 / 1973 serial number range: Q00,001-S60,000.
Beginning 1970: Production date printed on the upper left corner of the Safety Standard Certification Label, glued to the B pillar below striker plate.
I have no way to get info for a Domestic Special Order, but you can get the info for a fee from martiauto.com
Late 1980's: FoMoCo was planning to THROW AWAY all the 1967/79 records. Kevin Marti, a Mustang/Shelby 'nut' in Phoenix AZ found out...and got ALL OF IT!