KP, Thanks for giving this topic life !
Your topic gives rise to a question probably Bob Jameson can answer, "What were assembly line production numbers?" Seem to recall he was in Japan for approximately 45 days and left the middle-latter end of April, IF i recall correctly. Bob was present when vin #1 was completed two days before the production deadline of March 15, 1969.
I forgot this detail until i watched Ray's DVD last week.........One can assume with a high degree of certainty, there were complete machines coming off the assembly line after March 15. We also know the crankcase stamped 338 has (what we assume to be a casting date) of April 5. What we do not know, is what date production crankcase casting first began and how many crankcases began being cast after that date. (This is based on E338's cases being the earliest recorded vin having the earliest recorded casting date of April 5. Until otherwise revealed, vins prior to E338 had no casting dates on their cases.) It's safe to assume crankcase casting started some date in advance of March 13. We know the raw castings had to be sent from the foundry to Honda for setup for machining prior to engine assembly. We know the sand mold had to be re-created every time for every upper and lower crank case cast. We do not know what that lag time between raw casting to machined casting is/was. It's probably safe to assume each of these processes and procedures not only took a segment of time to complete as well as perfect.
Concurrent processes were producing frame compnents and fabrication of these components into completed frames, not to mention production and assembly of all the related parts ancillary to creating a complete motorcycle. On top of this, not to mention these ancillary components came from various manufacturers such as Nippon Denso, Stanley, Bridgestone and who knows what others such as fastener manufacturers, etc........ The point is, production would had to have been slower and fewer vins produced early on. Preparation and organization of all these factors would have been an enormous undertaking, just in and of it's self.
I cannot remember specific details of our conversations with each other as regards production dates of various vins, but i certainly agree with you, it would seem production numbers were lower beginning in March and higher in September.
I guess another way one could look at vins would be to determine if there are more casting dates occuring in a shorter period of time in later moths than fewer casting dates occuring in the early months of production....... 
Assuming E338's cases were cast April 5, perhaps one could hazard to guess these cases were assembled into a completed engine and fitted to frame 302, say, 10 days later.......
April 15...........
I am also hazarding the guess the painted frames came off one production line and the engines off another, each passed their own inspection, then were stamped, then taken to a general area where they were then somewhat randomly selected to fit a frame to an engine. Bob Jameson said in his presentation, because the engines were so heavy, the bare frames were slipped on to the engine which was positioned sideways on the floor to accept the frame.
I really enjoy this sort of speculation, sort of like guessing how many angels would fit on the head of a pin, but none the less, entertaining.
Would really b great if we could find a way to make a connection with our Japanese friends to learn more.............
