I would like to see a picture of the tag. For the vintage you are showing the tag has chrysler number then IBS-4015 for your 68 as example. Date codes are 2 to 3 numbers week and year. A letter may be after the IBS number which is part of the part number. For example a IBS-4014, 4014A, 4014B are all different distributors. An IBS-4014S is the Service over the counter distributor for the 4014. It would also have a different Chrysler number.

In the Hemi 6 pack world, there were changes to distributors that had the chrysler number stay the same, but the IBS number changed. Same distributor for the same model car, but some internal change. This created the early versus late model stuff.

Attached is a picture of a 70 hemi tag.