Production year marked on casted parts

While talking with Mr. Van Zanden from CITROTECH in the Netherlands about the revision of the front suspension units of my 1968 DS19, he mentioned that his customers like to have the original unit of their car repaired instead of exchanging them for already repaired units.
The reason for that is that the production year of certain (casted) parts is marked on the parts. This was new to me, but as you can see from the photos on this page this is indeed the case.

Figure 1 and 2 show parts from my 1968 DS19 which "Date de commercialisation" is the 25th of Jan 1968 according to official Citroën information. It makes perfectly sense that parts of this car were made in 1967.

Figure 3 shows the suspension unit of my 1958 ID19 which "Date de commercialisation" is the 10th of Jan 1958 according to official Citroën information. It makes perfectly sense that parts of this car were made in 1957.

Figure 1. Aluminium tripod housing of a February 1968 DS19, the number in the casting reads 67.

It would be interesting to know if the 3 digit number in front of the two digit year number indicate the production week or day of the part in question. It could be the day of the year the part was produced, all four numbers are within the 365 day range and would indicate a day more or less close to the end of the year, which would make sense for parts on car produced early in the next year. Maybe somebody can check this hypothesis on his car(s) ?

Figure 2. Aluminium suspension block (right side) of a February 1968 DS19, the number in the casting reads 345-67. The number on the left side suspension block reads 363-67.	Figure 3. Aluminium suspension block (right side) of a January 1958 ID19, the number in the casting reads 338-57. The number on the left side suspension block reads 297-57.

Photos by Jint Nijman