Great dialog! I have learned quite a bit.
As requested by Jerry, I sent him pictures of the stamps and date codes. He replied verifying the block is a real Z.
Still can't explain the date gaps, however. Someone told me that there might have been a strike during this period (May, 1968 and Sept, 1968), but you would think this gap would be much more prevalent in blocks that were produced then. Perhaps the bock needed some rework?
Anyway, now that I know it's a real deal (whew!), off to begin the rebuild process.
Thanks to everyone who contributed.