Forum:TOS Credits Still

There's a still in a few TOS credits scenes that I don't recognize. It's a big circular brown door with Spock and Kirk standing on it. Which episode is it from?
 * Do you have a specific episode this still is from? (by that i mean which episodes ending credits) The ones I've checked don't have the particular description you've stated &mdash; Morder 01:32, 7 April 2009 (UTC)