Without cross checking bores and strokes the date of 70-71 makes me think it is referring to the second of the special alloy Can Am blocks Chevrolet built.
Think they were 356 alloy same as ZL-1s
Specifically built for the Can Am circus, although Grumpy used a short stroke one (might have been the 390 alloy version ) in his 1970 Camaro. Later used by Group 6 Vette and Camaro racers in Imsa and T/Am.
Rare is kind of an understatement.
