Although based on the picture it doesn't really look like a crown or the three interlocking circles indicative of the Nirosta symbol. My guess would be it's the Nitro proof. If it makes any difference the proof appears on the frame, slide and barrel. You can faintly make out the proof on the slide in the first picture right above the proof on the frame. Although its more worn and the "N" looks like a "J". The final proof on the barrel is unfortunately lost in the flash.
