Bluthner grand

[Tunethepno@AOL.COM]

I just looked at a Bluthner "Aliqout-grand." (I'm not sure I got the spelling
correct on the brand or model, a little distracted by snow this morning.) It
is apparently pre-1900 vintage and needs rebuilding and refinishing. I have
two questions about it. I assume that the extra string riding above most of
the treble is kind of like a more proactive version of a duplex scale. How
does one go about tuning these? Just tune it to the unison it is mated with or
is there some more arcane usage? Any light would be appreciated. Also, the
owner was wondering about a value for insurance purposes and as this is the
first Bluthner I've run into in 25 years of Indiana tuning, I could use some
input. I've heard that these were exceptional pianos but there apparently
aren't a lot of people around here who have them. 
John Stroup
South Bend