The tin plate was found at the site near the parish church from where a monument to Mile Budak, a member of the Croatian World War Two Nazi-style government and writer, was removed late last month.
The tin plate was stored in the police station in Gospic, a police statement said.
The monument to Budak, who was born in Sveti Rok, was raised on August 21 at the initiative of Mr and Mrs Vidakovic, Croatian emigrants living in Canada. It was removed in the early morning hours of August 27 after the government had decided the night before that the monuments to Budak and Ustasha Black Legion commander Jure Francetic be removed.