That is the only VIN stamp on the block. You already know it's from a Firebird, and there is not enough data on the block stamp to tell coupe or convertible. The letter will be between the 2(Pontiac)9(1969) and the 104212(partial VIN). Yes, the missing letter will tell you which plant, and will narrow to two VINs. If you want to try to match the engine back to its original car, it might be possible with a bit of work, IF the car has ever had PHS documents pulled.