So it is interesting after doing some research on the 340 distributor. It does look like it used that unique tag from the examples I seen. I would add the IBS numbers to your list as the letter code after the 4015 helps with model year too. From you list I would say:
Z=67
A=68
B=69

Also, the 4015C 3438317 is the early 1970 distributor. The later version 4018 retains the 3438317 chrysler number but has the different IBS number as I mentioned. I would believe the number in your list is a month. Why they did this for the 340 distributor, when all the other IBS around that time was in a new format with different tag logo is beyond my knowledge.

I learned something new today.