The thing that is the main determinant of build date is the Oval Plate number rather than the chassis number. From my reasonably extensive research of around 400 odd cars, it would seem that the chassis are built in batches for the various markets.
For instance, to take an extreme case, chassis numbers for RHD 8 Gordini's are in a fairly tight range, however once you look at the larger production range in conjunction with the oval plate numbers, the oval plate numbers indicate the actual assembly date and equipment used. In the case of the 8G, LHD and RHD cars with close oval plate numbers can have widely varying chassis numbers, to the extent that a RHD chassis number can match up with a much older LHD car. The same pattern seems to be the same for RHD 15's and 17's. In a nutshell, the RHD chassis' seem to be made in one batch, then those batches were stockpiled and pulled out whenever they programmed to make a batch of RHD cars. However the AUS assembled 12's and 16's show a much more ordered pattern, there is a relationship between the chassis number and ovalplate number that does not exist with the French assembled cars. Apologies if that seems terribly geeky, I'm a sucker for all the various numbers.
Also sometimes (very rarely) I've seen the manufacture date stamped on the diamond plate like 2-72
The PR 953 parts catalogue quotes the following oval plate number ranges for the various model years. Note that the French model year, therefore the oval plate number, generally commences in September of the preceding year:
1971 model year R1120 1164581-1263200, R1123 931590-1122678
1972 model year R1123 1122679-1239156
1973 model year R1123 1239157-1357385
I know that all the above doesn't really answer the question of what is the actual production date, hopefully your car is a 1972 MY car from the ovalplate range that happened to hang around in the showroom :-). So as an addition, it may be worth a letter to Renault, they may have a service that provides a certificate of production for their "heritage" models.