With the 1G VIN, the engine is from a 1971 B-Body from the St. Louis plant. There is no way to know if it was an original Sixpak engine unless you can match the VIN up to the car it came from. Some people think the the HP2 stamping is for sixpak blocks, but this is not true.
There is a thread in the engine section I believe with a lot of data on the subject.
Barry