We have this same debate w Pontiacs the c118 RAII block is the only as cast 2506 block the rest were 9790071 and they ground off and stamped the rest of that 2506 number. this has led to a debate about that being a high nickel block. Some RAII race cars had that block moved to the next seasons car as they were both considered 2506 blocks NHRA allowed it. Speculation was that it must be Because it was high nickel . (The story the driver gives is it was already balanced and blueprinted and was considered the same block ) saved money.
A few months back I posted a few outward anomalies in the blocks even as cast on the same day . It was explained (by someone from a foundry) as the blocking in the molds for the dipstick tube was differnt and they had just grabbed 2 or 3 of those used on some blocks where others maybe used 1 of them . It is a very similar debate to what you have here w these blocks and around the same time period . M
Interesting coming from a segment of Gm and seeing the same basic debate .
This may help explain why some of the outward differences may have popped up over time with your blocks as well . Rember these differences can be on the same day of casting w the Pontiac block
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