WM21HA8319705 would have an SPD of about 616 ish (mid June). 1968 LR VINS and SPDs go to at least 346XX and 717. Only tracking one type of make, model or plant limits the number of data points. The more wider the scope, the more data points and stable data.
33-1 was released for the Charger in February
The 1970 Hamtramck Registry - 1968 Dodge PIBs The earliest example I have on file has a March SPD.
Your visual inspections and comparison are noble, interesting and academic as, in the end, there will be no way to document or prove in any manner what color the car was originally painted. Any claim of 33-1 without a tag, BS or window sticker will be a claim out of the ordinary and be viewed as another "car show anecdotal story". It very well could be another shade of red and coded 999 for paint...but that can't be proven with documentation.