Gary if you ever find that GM Directive, I would be interested in seeing it.
From what I have seen, big blocks at Tonawanda generally did not receive a letter in the CE stamp, because fewer were produced and thus in general there was not a need to reuse the numbers. There may be some cases where they were reused but I have not seen any.
Also from what I have seen, big blocks from Tonawanda received a stamp down on the pan rail near the oil filter area, which seems to be an assembly date stamp. This would be something like T097 - T for Tonawanda, 09 for month (September) and 7 for last digit of year (1967 in this case). I am looking for documentation of this. I am only going off of what I could determine from the blocks that I have.