I'm not going to be of any specific help on this one and we're going to need someone that has removed or found some originals on an unrestored car with the same suspension and year
MPC don't appear to be of any help since it appears that Ford, to keep inventory needs small, quickly used the one or two types fit all application quickly. After a few years they just carried the stiffer ones for all applications cutting further the need to carry multiple types. Because of this almost if not all of the NOS ones I have pictures of are of all the same part number for the rears
For a some what related detail it appears that for the fronts the C5ZF marking changed to the C6ZF in Feb 66 (shock not car build month) and for your car they were very likely stamped with the AUTOLITE name rather than the earlier FoMoCo in the oval.
Hopefully some others will have some collected information to share