Willy Köstler flatback

[Ron Wouters]

Hi,
I found a beautiful and good sounding German (?) flatback from around 1920, only i can't find anything about the builder Willy (Willi?) Köstler.
Anybody here can tell me more about him? I didn't buy it yet, because i'm not sure it's worth the money asked. If it is, i'll get it asap, so all info is very welcome before someboby else runs off with this beauty.

Thanks,

Ron,
Belgium