Abstract: A stylus is briefly placed on the surface coating of photographs for forming an artificial marking symbol on the coating. The stylus oscillates in the ultrasonic frequency range. Part of the oscillation energy of the stylus is converted into frictional heat producing a marking symbol in the coating in the form of a high contrast annular bead. The apparatus comprises a stylus, a mechanical lowering device, an electroacoustic transducer for producing the ultrasonic oscillations of the stylus, and a frequency generator, an amplifier and a voltage/current converter connected in series with the electroacoustic transducer. The transducer is supplied with a signal from the frequency generator, which signal is kept constant by a control circuit provided on the generator.