Interesting point. I once read that the city of Nagoya had 17 sword factories surrounding it. I know the Seki operation had many more than that. So, in once sense, it's surprising that there isn't more variation in the font than there is. But I guess it depends upon where, and by whom, the stamping was done.
I mostlycollect weapons ww2 German. Hence the Mauser99. Serial fonts are seriously studied in that side of the hobby as the forgeries are crazy. As for a factory its not uncommon for them to be changed as they did break or wear out. But, you see a gradual deterioration of the stamp then replacement. The font differences here could be two things. One a different factory or just two sets of stamps for different shifts maybe ? Or two separate lines in the factory.