I do not agree that we are 'back to square one'. I believe we've confirmed that Mr Serio is correct (NO DATE CODE, per se), BUT.. we've also learned empirically that the ID code/SN has a month/year encoded into the SN... which serves the same purpose for us.
A 'date code' is a marking put on a part purely to record the actual date (time/shift/etc) without any other purpose. The Hurst ID/Serial number code has multiple pieces of information in it, some of which we do not know, but we've pretty conclusively shown that there is a month and year encoded as part of this SN. But I doubt they did it just so we could demonstrate originality to a car 44 years later..