A man in Spalding for the Flower Parade dropped a knuckle duster on the ground as he was invited into a police vehicle in The Crescent, Spalding magistrates heard.
Rebecca Ritson, prosecuting, said police constables approached Ashley Robins (23) because he matched the description of a man their sergeant had asked them to stop.
Robins was polite and cooperative so police decided to invite him to sit in the police van.
As an officer opened the sliding door of the van, he noticed a slight movement of Robins’ left hand and heard a metallic object fall to the ground.
The object was a knuckle duster and Robins was immediately arrested.
Miss Ritson said Robins signed the officer’s pocket book to say he had dropped the knuckle duster.
She added: “There wasn’t any allegation of it being used.”
Magistrates fined Robins, of Birmingham, £100 and ordered him to pay £85 costs and a £15 victim surcharge.
They ordered forfeiture and destruction of the knuckle duster. He pleaded guilty to possessing an offensive weapon on April 30.
Solicitor Daven Naghen, mitigating, said Robins had seen two youths with the knuckle duster, asked them what they were doing with it and took it off them.
Mr Naghen said if Robins had handed in to police straight away there would have been no offence.