It is unlawful for any person to wilfully or maliciously remove, destroy, mutilate or alter the number of any bicycle from licensee pursuant to this chapter. It is also unlawful for any person to remove, destroy, mutilate or alter any luminous sticker or registration card during the time in which such luminous sticker or registration card is operative; provided, however, that nothing in this chapter shall prohibit the police department from stamping numbers or letters on the frames of bicycles on which no serial number can be found, or on which the number is illegible or insufficient for identification purposes. (Ord. 174 §6, 1972).