part (valve?) left in toilet

[truffaut]

hello a week or so ago we had a plumber at ours to service our macerator he exchanged a few parts and did some descaling. it’s all fine i guess but yesterday we found this piece in the toilet sort of like down below in the bowl just before it all disappears. i messaged the plumber but have not heard back yet so thought if i post the picture in this forum maybe someone know what it is? i just want to know if this is something that came off the macerator or just somewhere else. i feel like it looks like a part of a valve or something and maybe it got left in there when parts where changed out but i’m no expert. does anyone know?

 

[Geofd]

Remember, your in England we are not when you say toilet, I see the bowl but doesn't look familiar

 

[TomFOhio]

Looks like the gasket that goes between the fill valve and the bottom of the tank. Can you take a picture of the inside of your tank