Hi, Mjoely and welcome!

Your partner's accordion is a 26 treble key, 48 bass button 2 treble voice piano accordion, possibly Sette Zero made (Giulietti badged) entry level piano accordion from about the late 1960s to early 70s (?).

The basses are arranged in 8 rows of 6 buttons.
In each row, the buttons (starting nearest the bellows) are counter bass, bass, major chord, minor chord, seventh chord, and seventh diminished chord.
See here:
(This is how you would see them in a mirror while wearing the accordion)

Although a basic instrument, it should be well made and durable and suitable for a variety of uses including folk music and busking.
After some 50 years, depending on how it's been treated and stored, it may need some TLC(?)

Measure across the treble keyboard across the outside edges of the outermost white keys: about 17 inches, junior sized; around 19", adult sized.
It's quite a nice item!