I would disagree as it's actually more common to find 1937 1 fen in better condition rather than in worn condition. Manchukuo 1933 - 1935 1 fen coins are normally found in circulated condition however the lsat couple of years are different.
Due to the strategic importance of nickel and copper coins as war metal, these coins' composition were changed in 1939. Some of these coins were actually paid out higher than face value as the war prolonged. As these coins did not get to circulate too much, it is actually more unusual to see them heavily circulated. I've gone through at least 100+ of such coins and yet to see VF condition of 1937 - 1939 1 fen.