I have never seen a single example of a 68 with a trans stamping and no engine stamping. Not saying it never occured, but I'd say the chances are pretty slim regardless of what time in the 68 production year the car wa built. Also, I've had a pretty fair number of owners (some of them pretty knowledgable to boot) whom I've done personal inspections/verifications for who swore they had an original non stamped block only to discover that they actually did, they just couldn't locate them without help.