Re the VIN stamp on the engine.. Anything is possible, but not having one is not probable... 
I have emails regarding 2 and have seen an additional 2 DZ blocks which had been vatted, no paint on them, built between mid March and mid April with no VIN anywhere, not in any angle, any light, going so far as to invert colors and use filters in photoshop, obviously there were many Z's built during the period however to find between 2 and possibly 4 from my postings about this years ago illustrates they are around.
Perhaps the confusion resulted from the phasing in of Firebirds to Norwood from Lordstown in April of 69, can you imagine? Pacecars, COPO's, Z28's, RA Firebirds and then in May Trams Ams... Holy crap the number of parts on the assembly line would likely increase by 30% is that correct?
Those VIN stamps in the cast surface are EXTREMELY difficult to see and read, vatted or not. it took me a LONG time to be able to see mine, cleaned, using varying lights, etc before I saw one character; once you see a character the others begin to be seen (I think by our brain processors using correlation techniques
.. it took me even longer before I was able to get proper lighting, exposure, etc to get a decent photograph of it.
Also, I believe one could have gone to a Chevrolet dealer in March of 1969, and bought a 302 short block assembly (or a bare block); the pad would not be stamped with anything, but it would have appropriate casting numbers and casting date in the block...
but yes, it's possible (NOT probable) that one could have been missed.. people (most people) are human and make mistakes..
2 or 3 missed blocks for VIN stamping.. from how many thousands?... is a very improbable event.
