Help to ID very Strange 426 Wedge Block

This is identical to my block. It is definitely not a 413 block. I also have '69 and '72 413 blocks. Both blocks are clearly stamped 413 T on the pads and casting dates are clearly evident on the sides of the block. The other anomaly on my mystery block is that the bores measure 4.246 - 4.248. My "speculation" is that if these were intended for Nascar that they would be released unfinished so that the race team could have the cylinder walls finished to their spec.

Randy