Based on the link provided by OFG, it appears that the outer cog may thread onto a special cog, to convert a 6 speed to a 7 speed? So maybe Shimano re-engineeried the MF-7400, 6 speed into a 7 speed and dropped the MF-7401, 7 speed? Just speculating, as my literature shows a MF-7400, 6 speed and a MF-7401 7 speed.
The ratios still appears customized. Shimano's standard 12-21T, 7 speed product was 12-13-14-15-17-19-21T. The replacement of the 14T by an 18T is a new to me.