Before going to Chieftess Niamh but after giving the horn to Donnchadh, you should be able to talk to Revelin in the parthanan stable near Donnchadh to receive Revelin's Ring.
This item was definitely in the game in 1.65.
CTRL+F and search for Revelin:
Look at date last edited:
The full process is described here:
There are more sources on the net from sites in other languages but this should be plenty of proof.
Also, there are various features missing from the epic quest itself but that is for a seperate ticket.
1. Complete the Last Heir epic quest up until you kill Cailean and give the horn to Donnchadh
2. Attempt to interact with Revelin.
3. Fail to receive the ring.
Simply implement the ring upon interaction with Revelin when at step #16 of the quest.
Revelin's ring is in here.
Here too, check date edited:
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