Thanks to Flickr user der_spielmann for confirming that this is the interior of Pioneer's Palace in Moscow's Lenin Hills. On the right side is a sign with the logo for the IEC (International Electrotechnical Commission) 1969 conference.
Taken by my grandfather David C. Cook on a visit to Moscow for a meeting of the International Electrotechnical Commission. Development stamp is June, 1969. Unlabeled slide from a carousel labeled "Russia".