I am betting it is a High Nickel content in the block cast mix. The chevy 350 guys want the 010 block that has the high nickel content for their performance builds. I honed one of these and the Cylinder Walls are really really hard.
A good simple test might be to see if it has very little ridge in the cylinders at the top compared to a regular 318 cast block that usually gets a pretty good ridge at the top of the cylinder after a lot of miles.
Will be interested to hear what you find out more on it. Pretty Unique !
