“Unofficial era name” in Japanese
Short answer
Unofficial era name in Japanese is “逸年号” (itsunengou). Full meaning: unofficial era name, apocryphal era name.
Other ways to say “Unofficial era name” in Japanese
異年号
inengou unofficial era name, apocryphal era name
私年号
shinengou era name used by commoners (as opposed to the imperial court), unofficial era name
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