Thanks Ken. I'll get both KOSEI and KOKUMIN nenkin in Japan, so after reading your note I made some phone calls today.
First, I phoned about my KOSEI nenkin, and I discovered that when I retire, I will actually have 30 years and 5 months (365 months) of Japanese work credit. So my total Japanese (30+) and American (10+) years of work will add up to over 40 years.
Anyway, the KOSEI nenkin representative said that I have already qualified in Japan with over 25 years here, and therefore they won't use any information from the USA. And he knew nothing about a certificate for the USA.
I then went to see a KOKUMIN nenkin office representative. He said the same thing. With over 25 years on my Japanese work record, they won't use any American data. I tried showing him my US Social Security estimate, but he wasn't even interested. He, too, knew nothing about a certificate.
As for my US Social Security, an American friend in Japan whose situation is similar to mine, and who has started getting US Social Security, tells me that she had to submit this "Modified Benefit Formula Questionnaire" for the W.E.P. to the US Social Security Administration:
https://www.ssa.gov/forms/ssa-308.pdfand she said she also had to enclose the information from the Japan Pension Service about her pensions from Japan. So I guess in a few more years, when I'm old enough to apply to the US for Social Security, I will need to get some sort of "certificate" or proof from Japan about my Japanese pensions to show to the US (but not to Japan).
As for tax, it seems clear that Japan gets first right of taxation on my Japanese nenkin since I live in Japan. As you indicated, Japan grants a pension exemption up to 1,200,000 yen for people over age 65. But I'll get more Japanese nenkin than that, so I'll have to pay some Japanese tax.
For America, since I have to report ALL income on my US tax return, I also must report my Japanese pension (less an annuitized amount for the payments I made into the nenkin system over 30 years). So I guess I'll pay US tax on my Japanese pension and then I'll get to claim a tiny bit of US tax credit for the Japanese tax paid on the amount over 1,200,000 yen.
As for the US Social Security, I wonder who gets first right of taxation? Japan, since I live in Japan?? I kinda doubt the IRS would see things that way, and there's nothing I've found in the tax treaty that indicates I can take that position for pension money that comes from the USA.
My American friend living permanently in Japan says she just reports her Social Security to the USA but not to Japan. Somehow that seems wrong to me!
This is confusing, and I'm not the first person to be in this situation. For instance, there are many Japanese retirees in Japan who once worked in the USA, so there must be some clear and accurate answers out there somewhere. I can read and write Japanese, so if anyone knows of a Japanese-language forum about international matters such as tax and pensions, I'd like to also try posting my questions in Japanese there.