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One more example I found, for V13 for last name starting with V and date of birth 13 January (this time the number matches the day)
We need some more examples for assessing if there is a pattern related to day/ month of birth
Yes, we would need more combinations (name/ birth date/ code) to get some more flavor, maybe it was not a fixed rule, maybe sometimes he put the month, sometimes the day, etc.
I have found for a frame of JJ SIMMLER, born 21/06/1941 that it had stamped: S6, so the number matches the month - possibly a coincidence as this rule does not cover numbers >12