Ordovician gastropod moulds?

[mbarco]

Upper ordovician, n-e Italy. I don't have the specimen (no scale bar was provided). 

In the ordovician couldn't be an Ammonoidea (and I don't think a coiled Nautiloidea). So I was thinking about Gastropoda.

Fig.1 external mould. Central part removed.

Fig.2 internal mould.

Fig.3 external mould.