F body vin would start with a 5, so probably not applicable.
It appears someone used a tool, such as a narrow chisel, to stamp a small x in the depression above where the letter would have been. Maybe when the stamper realized the stamp wasn't complete ?
Again, more speculation...
The production numbers of Nova L78's would suggest the highest probability of it being an X.
Cost and location are out of my range, but it would be fun to have for my '66 Chevelle, so the CRT engine in it now can go in my '70 SS.