I have some experience of a variety of accoustic accordions, newer and older, and my impressions are that pre WW2 accordions were largely made of wood but postwar, as time went on, an increasingly greater proportion of metal (aluminium chiefly, but also pressed steel) entered into the construction: the framework, treble board/key bed, treble keys, pallets, bass board, grill, sounding board, all exterior bits being covered with celluloid. Scandalli, Settimio Soprani being early examples.
To my ear, accoustic accordions with a lot of metal in them have distinctly metallic/ tinny overtones as all the metal bits resonate, even the multitude of treble key springs/bass mechanism reverberate, even squeak . This can be quite noticeable, especially in the cheaper models.
A particularly annoying problem where external body work made of pressed metal is concerned, especially treble grills and their surrounds (the Armando Bugari Champion being one such example of many), is that, in cold weather, the accordionist's breath tends to condense and moisture collects on this portion of the instrument (similar to the moisture that collects in brass and bagpipes). This does not happen with older instruments where the relevant structure is actually celluloid covered wood.