I was going to do some digging before I posted, but the above post drew me in.
That info is correct for Camaros (except L72 could get M20's, I'll correct that). I'd need to do some digging to find the backing documentation, but the jist of it is the M20 wasn't available in a Z or L78 in 67 or 68. In 69, any Camaro could get an M20. Not sure of the logic behind it and have no idea on other models.