@sofj45 ,
Interesting info on the engine serial numbers, which may point to the assembly line alternately building 40s and various 45s coming down the same line. At some point it would be interesting to merge the various known 40 and 45 registry frame serial numbers as well as the engine numbers to possibly find an assembly pattern if one exists--perhaps a limited run of one model followed by a different model, or maybe individual models were on a made-to-order basis as orders came in. Certainly, with today's volumes of mass-produced vehicles, the largest manufacturers must limit the amount of individualization to not slow down their production rate.
Of course, the early assembly methods would have corrections as the volume of production ramped up over time and efficiencies were found. The "Just In Time" mantra would indicate there were not a lot of parts sitting idly-by, where consecutive numbering would get messed up. Also interesting would be if serial numbers on frames, engines, etc. were stamped right after their manufacture, and that certain batches were sent to the various overseas assembly plants during or after the model run.
Another question that comes to mind would be the difference between date of manufacture and date of final assembly, similar to the dates of importation, sale, and initial registration. If...................all the parts, pieces, and components of a vehicle were found years after their original manufacture, and someone then assembled the entire vehicle, what "year" would be assigned to that vehicle--would it be the date of the frame, the date of the newest component, the date that model last stopped original production, the current date, a date arbitrarily assigned by some registration bureaucrat, or something else? Certain rare and limited volume vehicles--antiques, exotic cars, for example--can achieve huge differences in value due to their "age", being the "last one made," and other uniqueness. There is at least one FJ45 out there that professes to be the "last made," having been assembled at a satellite assembly plant--on a date after the original Japanese production ceased.
I suspect we should consider it an outlier or unique oddity, rather than part of the original model run, and let the owner and any buyer reach agreement as to any increase (or decrease) in value due to a specific "date." <-------my very biased opinion.