Not 100% sure, but it looks like the MM cap used by JB Racing for their twin plug distributor.
The original MM parts are NLA, but there are usually some new versions available at places like Amazon.com.
For example,
https://www.amazon.com/Facet-2-8210PHT-Distributor-Caps/dp/B00DZMTASI/ref=sr_1_5?keywords=Distributor+Cap+jaguar+xj12&qid=1581045716&s=automotive&sr=1-5
I think the application is Jag XJ12
There are some PP threads on these. All of the current versions are likely made in the same factory and sold under brand names like Facet and Beck Arnley