I have two of these knick-great bases. Both are German fingered, both have the low Eb key for the right thumb and the open-holed F key which also allows for a good F# (usually a difficult note on a German fingered instrument). I found one (along with an Adler knick bass) at an antique shop in New Hampshire. The other I found online just a week later. They are fairly good instruments, the bass being better (and with baroque fingering). I don’t know if revoicing would improve them. They are made from maple and stained to look like pear. One of the great basses had a fingering chart in the case dated 1956. The practical range is up to A or Bb, but I have found fingerings up to D.
They were advertised in the American Recorder in the mid 50’s until around 1960. I played on Adlers in the late 1960’s and they were decent instruments for the price.
Charlie