Where was Letter Never Sent filmed?
1959 film by Mikhail Kalatozov
Letter Never Sent (1960) was filmed in Yenisei and Usa, 2 places named in its Wikipedia article, in Russia. Wikidata records no filming location for it at all, so the article is the only recorded source. Each place says where its claim comes from.
Places per Wikipedia's article
The Wikipedia article names 2 places. Each links to the sentence that says so.

