I beleive 6 is correct for Caddilac because the LT1 came in a caddilac in a D car. 1 or 1B would be Chevy. 4B would be Buick and 6D would be the Caddy. So number is Make and letter is body.
5.7L SFI (LT1) (VIN = P) 1995 1,4B 6D - CARS
EDIT: Vin = P I beleive is for engine RPO from an earlier link. But Vin = P was used on 2 engines...

P LT1 5.7 L V8 SFI
P LSJ 2.0 L SC Straight-4|I4 SFI


Could 5 be Holden?