Another interesting tidbit that may be interesting in this discussion. 20 years or so ago I bought a BHJ ultrasonic cylinder wall gauge. It came with a handful of reference pieces to calibrate it for different cast iron alloys. One of them (designated L-3) was for Pontiac 1965 and later, Olds 1967 and later, GM corporate V-6, Caddy 1968 and later, All V-8 Buicks, and all AMC V-8s. This was not the same for Chevy high nickel/bowtie nor of course for regular chevys.
From this one could guess that in 1965 Pontiac changed to the Buick alloy, followed by Olds in '67 and Caddy in '68. There was no standard provided for Pontiac '64 and earlier and I have always wondered what the difference was.
I have a 64 and 66 block sitting next to each other. Maybe sometime I will get t a chance to wire wheel them both and look at the shine.
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