This gasket is not offered by the OEMs for sale, and is not included in “complete” carburetor rebuild kits. If the accelerator pump nozzle gets clogged in an old FCR carburetor it is nearly impossible to clean it without taking the middle of the carburetor apart. It’s a bit tough to get the gasket and O-rings to stay in place so you will have to use an adhesive similar to what the factory uses.
If you have problems starting or running an older MX bike with the Keihin carb then this is a must, installing the normal carb repair kits without this is just doing half a job.