Hi All,
I've been after a plunger for a 664. I was resigned to having to design and 3d print, but I noticed the Amiga 500 plungers were also by Mitsumi and similar, so i bought a few.
It fits, although the central hole is a little larger on the amiga. A bit of blue tack and all is good.
Hopefully this will be useful to someone else.
664 plunger in blue in pic.