I just re-visited my literature and noted that the original MF-7400 freewheels had a minimum 13T. Based on the body diameter, it doesn't look like they could handle a 12T. Given that the 12T cog appears to sit outboard of the freewheel body and appears to be screwed onto the position 6 cog, it really does look like Shimano adapted the old MF-7400 body to a 7 speed capable of handling a 12T.