This North Atlantic Joint Business is a full disaster from a casual passenger point of view and even seasoned frequent flyers have difficulties trying to figure out their baggage allowance and other perks.
AY has effectively hidden the information inside their pages, but I found this:

https://www.finnair.com/pl/AYPortal/...E=IT&PAGE=ATJB

It is from March 2018 and on Italian pages and search on AY pages will not retrieve the documentation. I think I have seen a discussion about some changes to this, particularly with BA but this should not concern OP here.
There is a link to AJB Basic/Light fare and on page 5 a table with seat selection and baggage allowances stating that checked baggage allowance is 1 BUT only to AY+ tier members plus OWE on AY marketed flights. Other Oneworld Sapphire/Ruby members are explicitly excluded.
An extra bag is 45€/55$ one way if bought online and 50€/60$ at the airport.