I received a reply in another RC forum. I'll put the size of the baffle here so any future searches for the baffle size will get an answer.
This baffle is for the OS 855 muffler used for OS 160 FX and other OS engines.
The baffle is installed between the back part and the center shiny part of the muffler.
The plate is 2mm thick, 47mm outside, with a 14mm hole in the center.
Greg