Air Arms firing valve body s507 for the S400 and S410 air rifles.
Old and new version (pre and post 2013). Please compare with pictures.
Includes internal valve seat face – if you have replaced the valve stem and are still getting small leaks down the barrel this part is often the cause.
Easiest way is to look at the underside and see if there is a hole in the corner of the flat part where the gauge mount goes – see image below: