what tool to open this?

[Hashtag246]

Hi, got a porcalenosa mixer shower that i need to replace the valve on it. Not sure what tool is used on it as it looks unusual. please see photos with details
many thanks

 

[Timmy D]

Not the approved way but....either grab the outer edge real hard with some grips to ensure you don’t slip and mess up the thread. If that’s not possible, stick a flat screw driver in to one of the grooves as vertical as possible and hit it with a hammer to get it moving.