Attached is the original text of -
Calculation of a symmetrical airfoil with the lowest pressure position afterward
by Dr.Ichiro Tani and Dr.Chikanori Noda (Tokyo University 1940)
(最低壓力位置の後にある對稱翼型の計算 谷一郎・野田親則).
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No, it is not symmetrical. But they used symmetrical sections for wind tunnel tests, so it probably uses one of these symmetrical thickness distributions, wrapped around a camber line. It is mildly laminar flow at best, but still quite conservative.
As it happens, we have a J2M-3 Raiden at the Planes of Fame. I can't identify the airfoil. but I could probably do a thickness to chord measurement in the near future.
If I do, I'll do it near the root and near the tip.