The Flash 253 (1976)
Although Swamp Thing does not appear in this issue a page is dedicated to him and the upcoming release of a two book compilation comprising issues from series 1. DC released the collection after the series was put on the shelf in September of 1976.
I love the Willem Dafoe looking villain on the cover.
I’m sure most folks would say this should not be considered a Swamp Thing appearance and I can understand that but the publishorial page (which I don’t think is a real word) features the ST font/logo and the page provides some interesting background information as well as series information.