The CE Warranty Replacement program started in late 67 in conjunction with the 5 year powertrain warranties. It outlived the 5 year warranty though.I do know on some of the later engines they had a a warranty or service replacement code stamped in them,don't know if that was something the factory did or if it was something the installing dealer did though.
I thought (don't try this unless you know better) that old stampings can be illuminated/made more readable with nitric acid.
Thought that's what I heard crime labs use on guns when the serial number had been mangled, ground off, stamped over, etc.
Anyone have experience with this?