Revisiting this one (because the 1977 US reissue came up as random), the CBS matrices put this in about March 1971, shortly after Blood, Sweat & Tears' "Lucretia Mac Evil" single. It might seem odd that Kinney (as was) took a few months to rerelease a hit album like this, but perhaps Festival had a sell-off period for their issue.
Originally distributed by Festival as SAL-933,615 (entered the album chart the week of 5 Jan 1970). The Atlantic license seems to have gone to Kinney around the end of 1970, because Zep III debuted on the album chart the week of 21 Dec 1970 with catalogue number SD 7201, so this could possibly be a late 1970 or early 1971 issue - it might be possible to pin it down more precisely using the matrix numbers, which indicate Australian Record Company/CBS pressing.