You may be in luck, Peter. ATE plastic caps identical to your broken one were used on Volvos, many English cars and some German ones. Most small-sized Girling caps also fit. Look through your local breakers yards.
Over time (45 years!) the plastic goes brittle no matter what you do and will shater while taking some caps off, so I recommend buying an aluminum example from Pegasus Auto Racing in Wisconsin. The one I bought was $7.49 (p/n 3549). Pegasus is on-line at [www.pegasusautoracing.com]
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