Simplex 2099-9813 nut size?

I have a simplex 2099-9813 trim plate for the 2099 series pull stations I got a long time ago. I have an issue attaching it to my pull stations because the nuts it uses are an odd size that my local hardware store doesn’t carry. Does anyone know what size nut the 2099-9813 takes, so I can find them and order some, thank you.