Kinbote’s commentary 149 mentions two islands, “Nitra and Indra” (meaning ‘inner’ and outer’), “two black islets that seemed to address each other in cloaked parley…”
I believe these refer to two Hindu gods, Nidra and Indra. Nabokov may have used a different transliteration for Nidra (Nitra), or he may have decided to play a little word-golf in order to obfuscate the meaning in order to have the reader dig deeper into the fact that ‘Nidra” and “Indra” are anagrams.