Update: I have managed to work out that the date of certification is August 16 1970. The date on the certificate is a Showa date, Showa is part of a system of dates are based on the rule of Emperors, in this case Showa indicates a year is the reign of Emperor Showa or as we knew him Emperor Hirohito. 2016 is Heisei 28, after the current Emperor.
This date matches the condition of the box and sextant and is consistent with other Japanese sextants of a similar vintage. It also matches the first to digits of the serial number. This Yamatar sextant made by Taiyo & Co. LTD is almost identical, save that this has a brass scale and is lighted, it was for sale here at the time of writing.
I stumbled on a great deal on a sextant. The person I bought it from had inherited it some time ago and had no information on its origin. It is made by in Japan by Iwasaki & Co., Ltd, I have not found any reference to Iwasaki sextants on the internet other than the ad for this one.
The label, partly in English seems to state it was calibrated on August 16, 1945, the day after VJ day. It is hard to imagine why a Japanese company would have English on a label at that point in history.
Iwasaki seems to be a common name in Japan, the founding family of Mitsubishi is Iwasaki and this family controlled a huge segment of the Japanese commercial cargo fleet at the outset of the war. It seems possible this might be an in-house instrument company. Alternately the Iwasaki Electric Company seems to have the right type of history, but again no reference to sextants. More research seems to point to a number of small companies building sextants in the Tokyo area in the 60s and 70s.
The sextant seems to be very well made, it came with a 4x main telescope and a 12x. The 12x has an inverted image, but I imagine it will be great for star sights.
The drum does not have a vernier, but it is large and easy to read and I think I can get about 0.2′ precision on readings. The clamp and drum work really nicely and very accurately set up. The overall condition does not match an instrument that might be over 70 years old. is 46 years old.
I could not be happier with my purchase, but I would love to find some more history of this sextant.