HELP what goes on here????

[Nantucket]

Just moved in what goes on the shower drain? Do i need to buy whole thing or can i just get the top bit??
Thank you

 

[Aquarius]

Looks similar to this, but try and send photo to McAlpine for confirmation, unless @ShaunCorbs knows? McAlpine Shower Trap 19mm Seal 70mm Mushroom Flange White - https://www.toolstation.com/mcalpine-shower-trap-19mm-seal/p85309?store=AH&utm_source=googleshopping&utm_medium=feed&utm_campaign=googleshoppingfeed&gclid=CjwKCAjwgdX4BRB_EiwAg8O8Hf6JdCT2jBcJpZZjBVWxWIJsSVqu1rlJEhy_4P_RspPy2Jmsc7tkDRoCV_YQAvD_BwE you can get a mushroom flange on its own, however when I look at them the photo product seems larger.