Is a lid screw necessary on a concealed toilet cistern

[Gary111]

Hi, The flush won't working, I think I need to replace the siphon.
With it being a concerned cistern the current siphon has a thread that it screws into.

I'm hoping to replace it with a demountable siphon so that I don't need to remove the toilet pan next time it goes.
But it looks like they don't have a thread on,I'm wondering if it's really necessary for the lid to screw into it?
I guess it gives it support.

Thanks

 

[Gary111]

Also does anyone know the make model of this from the picture please?

 

[Madrab]

Not needed, as long as the lid sits onto the cistern relatively securely, it just there to stop debris and dirt getting in but been to many where the lid is now long gone.

No real chance of guessing the make of cistern TBH though Thomas Dudley and Wirquin were well known for them