buddan8, I think that you may be interpeting the tag a bit off. If I remember correctly all of the assembly plants all have an intial. Maybe the body came out of a Fisher plant, went to an assembly plant and was matched up with a chassis bult there or somewhere else?, just AWAG. Was the number stamped in from the back side of the tag (raised letters)or stamped in from the outside of the tag (depression)
P.S. have you checked out this site"
Chevrolet Cowl Body Trim & paint tagIt may help explain your problem
A real trim tag from a 1966 Impala 2 door sport coupe built in ST Style,assembled at STL (Saint Louis plant), and inspected by inspector with the number of 17
Note: The assembly plant is ST Saint Louis on this tag,
The 17 is stamped into the tag from the outside, it is often an inspectors stamp.