Yeah, I was somewhat surprised by those. "Classes" on domestic flights in the USSR?! And yeah, GUM was (and still is) a department store for Soviet citizens - https://en.wikipedia.org/wiki/GUM_(department_store).
It has nothing to do with air transportation which has been developed in Soviet era, where everebody was equal.
edit: minor mistakes.
https://ru.wikipedia.org/wiki/%D0%A1%D0%BF%D0%B0%D0%BB%D1%8C...
https://www.ozon.travel/help/railway/preparation/trains/
As far as I understand, М has private bathroom but СВ doesn't. It's confusing but I'm sure I saw them sold at different prices.
There were no bathtubs on the trains in 1980-s in Soviet Union...