Attached is the info from the Aug 1968 MPC. If you went to a dealer on or after this date and wanted a replacement for a C8ZF-18080-G (or G1 I suppose) they would have sold you a C5ZZ-18125-B. What we don't know is how the actual shock barrel pulled out of the box labelled as C5ZZ-18125-B would be stamped, nor if each shock pulled out of C5ZZ-18125-B labelled boxes would be stamped identically, especially as time marched on.
Generally speaking it seems that Ford spent a lot of time and effort engineering systems comprised of dozens of variations of shocks and springs, only to turn it around and service them all with a handful of parts. Granted, if you went to a third party shop, there might be even more of a one size fits all scenario. It is absolutely a matter of inventory cost.
I think we have found that only by observing original parts associated with a known vehicle can we figure out the correct applications. Since shocks are a wear item it can be difficult to figure these out. We would need to hear from owners of various 68 Mustangs to see if this part number was used, and then look for patterns. It would be interesting to set up a survey and develop a spreadsheet to identify known original shocks, with the corresponding vehicle configuration and assembly dates.