Richard,
here is how I know it:
In Germany, the term Model Year does not exist, only the Registration Year. The year of the car is determined by the date at which the first owner registered it and received a title (“Erstzulassung”). If a car was produced towards the end of the year and first registered in the next year, it was a next year Registration Year.
In the US, the Model Year is governed by Federal regulations, mostly by the Environmental Protection Agency with their exhaust emission limits. EPA rules state that production for a Model Year can occur in any time frame which includes only one January 1. Therefore, as an example, the maximum time frame for production of Model Year 2004 vehicles is from January 2nd 2003 to December 31st 2004. This time frame has only one January 1st, in 2004.
In order to comply with this rule, but be able to headstart the production, games like the one you described were played, although only on very limited occasions. I am not aware of any such occasions regarding the W113.
Irregardless from the maximum possible production time frame, manufacturers certify their vehicles for a specific Model Year and inform the EPA of its start (date and serial number) on an annual basis.
As described in a previous post on Yahoo, regarding your specific vehicle, the Brown Beast, the 1971 US Registration Year began with registration date of October 1, 1970 plus serial number 113044-..-018506. Your serial number should be higher than that.
Klaus