
My thoughts started wandering to el Degas and the misery of not having any information on serial number interpretation.
For a long while I have noticed that there seemed to be a lot of MIJ acoustics with serial numbers beginning with "4" so I thought I'd have a short browse through the site and see how many I could find, and if there was any obvious connection between them.
This is, by no means, an exhaustive list but the preliminary results are encouraging and I think I may have figured out a small piece of the puzzle.
These models relate to Japanese made acoustics only, all obtained in roughly the same time frame (1974-76).
GB 25 Gibson Dove 4020145
GB 23 Gibson Hummingbird 4070134
GB 25 Gibson Dove 4080087
MT 18 Martin 4090021
GB 25 Gibson Dove 4090370
MT ?1 Martin 4100035
MT 18 Martin 4100086
GL 35 Guild 4100122
GB 26 Gibson Sunburst 4120316
The pattern I believe I am seeing is this:
First digit =The Year (1974 in this case)
Next 2 digits = The Month
5th 6th 7th digits = Monthly Production Number
and the 4th digit? = wishful thinking in case they had orders for over a thousand!

This is similar to Matsumoku's numbering system in the same era. It would be great if these were made at Mats, but it's still anyone's guess who. Chushin Gakki appears to have been involved in the solid body electrics, particularly the LP models, but I don't know for sure if they made acoustics.
Whoever it was they made fine instruments and I would submit that based on the above, that Japanese made models bearing a 4 prefix serial number at least came from the same plant in the same year.