Delightful, Wilfried!  Thank you for the additional information and images.  1500s...!

It seems there have been hummel instruments in aristocrat manorhouses in Germany. There is a plasterwork in the summer castle of the Duke of Mecklenburg from 1760 in Ratzeburg (between Luebeck and Hamburg). Many instruments of that time in good realism were shown as plasterwork in the rokoko music hall of that castle. To my great pleasure there is a hummel in connection with a bagpipe.  I took measurements this year and made a copy with cherry wood. String length is 66 cm.

This is the copy of the oldest Hummel in Germany. The original is at "Gut-Damp" at the baltic sea (aristocrat manorhouse). Its inside of a plasterwork from 1698. It was implanted as an old instrument with several broken parts and wrong situated strings. So I think this instrument was built around 1630. There is an interesting part on the end of the fretboard: Behind the last fret the fretboard from hardwood was thinned to avoid scratchings from the pick on the top. There are several Hummel instruments with nearly destroyed tops by the pick. This thinned part could only be made after looking at older instruments with scratches. Well - the oldest mention of a Hummel in Germany is from 1508. The Hummel was played in a religious walk together with other instruments. There must have been enough knowledge about this instrument in 1508 - otherwise they had written : A new and curious sounding instrument with the name Hummel could be heard.