Above quote was posted by KP in October 2015, but I found several references where the number 4 is used in an abbreviated form of 44 from the Year of the Emperor's reign. My opinion is the stamp on the tank of 4-6-2 = June 2, 1969. Others can weigh in.
I can live with that about the oil tank dates. Makes good sense. I think the Petrol (jumping jack flash) tank is 15 May. The tank is correct for mid/early and late Sandcasts so no biggie either way. I suspect it is the original tank nevertheless. My first exposure to numbering was with Kawasaki numbering and I just adopted this with the Honda numbering some years later. Certainly the 4 represents the year 1969. What seems to occur is that some parts have a year and month reference whereas other markings seem to have a month and day reference. Kawasaki have a mixture as well. For example they seem to have year and month as their dating but they also use the Showa system on some parts.
Really a bit of a dogs breakfast IMHO. If there is one thing that is consistent and that is numbering systems seem to be the same for part items ie: tanks, switches, engine parts etc. They use a similar format for each manufactured part
I would be interested in other thoughts. Chris may have thoughts.