It's time to change the rear cabin filter. The catalog number given is 87139-50050. On Exist in August 2022, it costs 2860 rubles.

Фото в бортжурнале Lexus LS 600h. Запчасти на фото: 8713950050

Upon installation, it was discovered that this purchased filter is much longer in size.

Фото в бортжурнале Lexus LS 600h

After a more thorough search in catalogs and with Exista specialists, it turned out that there are at least three types of such rear filters for LS models.
And the one I have is the most expensive, with the number 87139-5090, which costs 8000 rubles. There's also a third type 87139-5080, as established in the chat regarding LS models.
By measuring the dimensions of what I bought and the old filter, it turned out that they are exactly the same width and height, only one is much longer.

Фото в бортжурнале Lexus LS 600h

Фото в бортжурнале Lexus LS 600h

Фото в бортжурнале Lexus LS 600h

Фото в бортжурнале Lexus LS 600h

Therefore, with a slight knife cut, we transform a 2800 rubles filter into an 8000 rubles filter.

Фото в бортжурнале Lexus LS 600h

Фото в бортжурнале Lexus LS 600h

Considering both are originals and such a huge price difference isn't justified by logic (the smaller size costs more), the solution itself is quite successful and rational. Maybe someone else with the same rear filter as mine will find it useful too.

Фото в бортжурнале Lexus LS 600h

To everyone: smooth roads ahead, and clean air in your car's cabin!