I agree that the "516" next to the "Made in Japan" is part of the manufacturer plate and doesn't change. Maybe it's a code for the OEM factory.. or... well... who knows? It's not part of the serial number. Two of my M90's are Japan-domestic Victors. Their plates don't have the "516" on them, and their serial numbers both begin with 1464. It's possible that the 1464 denotes the Japan model, and the last four numbers (0330, 0803) are the actual unit numbers in the series.
Here are my serials:
1) 07840178
2) 14640330
3) 14640803
(Good luck, Gordie, finding anyone at JVC who has info on this. The original manufacturing facility might, but it's been so-o-o-o long ago. I know from toy makers that they generally don't keep these sorts of records for posterity. Plus, with JVC's new owner [Kenwood], who knows what, if anything, has been preserved from JVC's distant past!)