Dave,
Thank you for your post. It confirms what I had thought was the case on blocks/heads but I have no supporting documentation.
41Chevy's comment about Os and Is tweaked my unreliable memory regarding that practice - I think I now remember where I read that
Is were not used in AC Delco Remy (Division of GM) date codes:
example 1 (for Is);
example 2 (for Is); and,
example 3.
