The long explanation is that a one criteria for a restricted firearm is defined as:
It doesn't say a firearm has to be over 26" to not-restricted, it specifically says when the length is reduced from greater than 660 mm (26") to less than 660mm that makes it restricted.
A fixed stock Mare's leg that has its OAL less than 660mm is non-restricted because it doesn't meet the criteria to be prohibited (not a handgun, not a full auto, not a named prohib) or the criteria to be restricted (not a handgun, not a semiauto center fire with a barrel length less than 470mm, not a firearm that can be fired when made shorter than 660mm, not a named restricted)