The issue is with the plural handling of "accessory". it was a nasty suspicion for many hours....
This issue arises rather frequently. Pony up and come up with a name that cannot be interpreted mistakenly by rails or someone, somehow, somewhere. Avoid nouns that pluralize irregularly. Moreso when dealing with foreign languages.
Now
class Accessorio < ActiveRecord::Base
has_and_belongs_to_many :lavoraziones
has_many :accessoryvolumes
has_many :bozzas, through: :accessoryvolumes
and
<%= accessoryvolume.accessorio.nome %>
runs as expected