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According to the installation instructions on JP Enterprises website, it is a shim. If the silent captured spring buffer has room to rattle between the end of the tube and the retainer pin, you put the shim into the bottom of the tube before the buffer.
The washer is meant to be dropped into the tube first before the jp spring assembly. IF there is no tension from the assembly to start with. It takes up the slop if any from the spring assembly. Hope this helps