Nothing for sure, so take it with a grain of salt. But my research so far:
The mon is from the Watanabe clan. They were retainers of the Mori.
To note specifically is that the helmet is dated, 8th month of Daiei 3, if the inscriptions can be considered as correct.
Correctly calculated it seems that the 8th month in accordance with the Japanese way of calculating, would actually mean Oct. 1523.
Anyway, 1523 was the year that Mori Motonari became Daimyo. More in particular, he became daimyo after a succession crisis when the jong daimyo Mōri Kōmatsumaru died in 1523 and because he was nominated/asked by 2 of his vazals, among whom a certain Watanabe (found different first names) in July of that year. The official nomination of Motonari was signed by 15 of his vazals. Motonari seems to have entered Koriyama-Jo somewhere at the end of September/beginning of October. After that there was a period of trouble, where some of his vazals seem to have rebelled against him (among which the former Watanabe, who was executed in 1523 as a consequence of a cleansing operation undertaken by Mori Motonari because of the rebellion -https://mouri.gozaru.jp/monogatari3.html - these are recounted stories which were written down at some point so the actually amount of truth is under debate)
His son Toru was allowed to leave but was later pardonned and returned to the Mori Clan. He saved his lord during a battle somewhere in 1543 by posing as him and returning in the battle to allow Motonari to escape. Subsequently most Watanabe's received important posts in the Mori Clan and were remembered during the Mori Clan's New Year's Kachu Kaiki.
However, Nobuie was in Eastern Japan while the Mori and Watanabe's were in Western Japan. The mon could be a later addition, who knows for sure. Don't know if new helmets would travel all over the country. So anything is conjecture at best at this point, but the coincidence in the dates is remarkable if you ask me. Theoretically it would have taken a helmet ca. 30-35 days to get from Nobuie to the Mori domain. So it is possible.
Anyway I haven't overpaid it, so in the worst case I own a nice helmet with a fake signature but at a price which is acceptable even when it is not what it is supposed to be.
I have asked the Mori museum if any records would exist about an exchange of important helmets which could have been given to the Watanabe's in relation to their initial support of Mori Motonari.
