I agree with your observation regarding the scarcity of uncirculated Manchukuo coins. It appears that the only certified ones that are occasionally available these days are the 1942 five fen and the 1942 Chiao. I think these two account for almost half of all certified coins.
Do not know a source for problem free Manchukuo coins and it may very well be that there isn't one.
I also collect French Indo-China and Hong Kong coins which are also becoming nearly impossible to find MS certified except for a few dates.
I wonder if there were many collectors back in the day. Perhaps few examples were saved. A dealer in Korea once told me that many Manchukuo coins exhibited evidence of fire damage, something he attributed to the ravages of war. I don't know if that's true or not but it makes an interesting story.
You can find MS state Manchukuo coins on ebay once in a while. In fact, the price is not high at all compare with other Chinese coins. I think Manchukuo, Sinkiang and Tibet coins are under value right now compare with other Chinese coins.
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Do not know a source for problem free Manchukuo coins and it may very well be that there isn't one.
I also collect French Indo-China and Hong Kong coins which are also becoming nearly impossible to find MS certified except for a few dates.