Mechanical horns and "wind" instruments normally do not produce a single pure tone the way an electronically synthesized tone would be generated. They produce a range of tones.
In this case the regulation appears to be calling for a uniform level of loudness, across a range of tones that span 1/3 of an octave centered about the primary tone of the "horn".
An octave is defined as ". A series of eight notes occupying the interval between two notes, one having twice or half the frequency of vibration of the other." So if one tone is 440Hz, one octave up from it would be 880 Hz and one octave down would be 220 Hz. The regulation appears to call for a minimum volume level across 1/3 of that spread, i.e. if your horn was centered at 440Hz the volume might be measured 75 Hz lower (down to 365 Hz) or 146 Hz higher (586Hz). Assuming that's how they figure 1/3 of the "octave" range on either side.
If the manufacturer can't state that the unit complies with regulations, you can assume that it DOES NOT on any reliable basis. Save the math, unless you also plan to go out and conduct test measurements on the horn after it has been installed.