Mike Drew recently posted some clarification on the month code as there has been conflicting information put out in the past;
"The month character seems rather random at first, until you really study it. For some unknown reason, De Tomaso chose to use a 16-character repeating cycle. There is a typo in the Ford shop manual (Group 10, Vehicle Identification Codes) for April of 1971. It shows the letter "S" when in fact it should be "E" (as confirmed in the other publications).
The 16-month repeating cycle is: C K D E L Y S T J U M P B R A G."
So the month is not always the same letter because it's 12 months over a 16 letter cycle. You need to work forwad from 'E' as April 1971 repetitively until you reach your year and letter.