Peter,
I think I found that 9100 in an article written by Bob Adler (the expert on 47-55 Chevrolet trucks). I'll see if I can find the reference.
9000, 9100, 9200, and 9300 are all PPG codes for a black. 9300 is Jet Black (the DUPONT code is 93-005). As I recall, PPG did not have a modern formula but that might have changed. At the bottom of the link I gave above is information on how to contact the PPG historical color library. Ask them if they have Jet Black for 1951 Chevrolet Truck.
