@dave Sorry, I think I was wrong about the instance actor. Mitra discovers actor by analyzing the keyId parameter in Signature HTTP header. If the value of this parameter contained URL with a query such as ?id=920666, it was removed, so only https://ap.podcastindex.org/podcasts remained.
I explained that in a subsequent reply:
https://mitra.social/objects/018d1433-67e4-028f-79b8-7ced5a7bad0c
The address of public.mitra.social instance actor is http://public.mitra.social/actor