The Hohner Polyphonic 400 was built in 1975 and is a 4 voice double octave free bass convertor piano accordion, in C system.
The freebass keyboard is in a more unusual position using the Dominant 7th, minor and major buttons, the diminished 7th row is silent once converted.
4 voice double octave 41 key non cassotto right hand with a full 120 bass left hand. 5 left hand registers, 4 give stradella registers, the fifth gives the free bass keyboard
This is second hand and has certainly been played over the years. The bellows are showing significant wear. However a modern equivalent would cost many thousands of pounds so this is a great way to get in to freebass accordion