Batman #280 (1976)
The Only Crime in Town
Like many comics that came out around this time, this issue features an ad for The Amazing World of DC Comics.
Within all of those ads is a little Swamp Thing
Not only does this issue feature an ad incorporating an extremely small Swamp Thing, it has a familiar looking explosion.
an explosion, ripped from the pages of Batman 280
a similar explosion from Swamp Thing 1 (series 1)