This topic picks up on a mini-conversation that I started on Grannyknot's 280Z build thread. These little washers are an important part of the NVH isolation design for the Z's rear diff.  The design, from an engineering perspective, is a lot more sophisticated than meets the eye, so there's an incentive to try to install a proper replacement if your own have deteriorated (which they probably have, if they're 50-year-old originals).  Unfortunately, the part is NLA from Nissan and none of the