I have visited Greece a couple of times and have seen both these types
of pianos as well as the more standard street pianos playing in the
streets, mostly in the commercial sections such as the Plaka in Athens.

At the time I recall my cousins referring to them as being played by
Gypsies not Turks.  They said the same about some of the street
musicians who often played clarinets and would send a child from table
to table in outdoor cafes to ask for money.  So, perhaps Christian, you
should find someone familiar with Gypsy music as well.

Best wishes,
George Glastris