Detective Comics 426 (1972)
“Killer’s Roulette!”
This comic was released three months before Swamp Thing #1 in 1972.
I was surprised at how daring the subject matter was but the ad below was what drew me to the book.
The ad can also be found in Superboy 189. I’m on the lookout for other comics that feature the ad.
Detail:
rootsoftheswampthing.com 
I use to be the manager of a comic store and the owner has been in the business for almost 30 years now. He told me that a lot of times what DC would do is use the same ad in all of the comics coming out in a particular month. This Detective Comics #426 came out the same month as Superboy #189. So if you are looking for this ad in other books, it would most likely be in other issues that came out in August 1972.
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