If you've been searching for 7 string zither banjo's you will have noticed that the 7th string usually runs through a tube under the fretboard from the 5th fret and is tuned at the headstock.
Your banjo has the tuner at the 6th fret which sudgests to me this is a 7 string not a zither banjo.
Looking at the body, it doesn't look like it's been altered and the anchors for 5 strings could mean that the neck is not original to the body.
Difficult to find a specific maker, (there were a lot of makers) as there are no real distinguishing features except maybe the 7th string being at the 6th fret and not the 5th which seems to be the usual place.
They sell anywhere from £50 up.
to some other examples for sale in the UK.