Dr Strangelove: "While tampering with our water supply would be a good way to deal a lethal blow to major population centers in the United States, Ripper comes to the conclusion that the Russians are doing this because when he tried having sex his 'essence' was denied exit from his body." It's a bit murkier than that. Ripper says, "I do not shun women, Mandrake, but I deny them my essences," so why would he complain of non-exit? The water thing is about purity, a metaphor for cultural purity.
There's a story by Lord Dunsany called "Why the Milkman Shudders When He Perceives the Dawn" which... I dare say... seems to imply a conspiracy without actually going into any detail about it, save that five (and only five, "greybeards all and milkmen from infancy") men of "the Ancient Company of Milkmen" ever tell the story mentioned in the title...
Should there be a distinction between groups who use their identity as a cover say for example the milkman are really mafia who pretend to be milkman and those who actually are milkman but murder people to keep their monopoly?
I assume that if the Milkman's Conspiracy is to deliver poisoned milk or spoilt milk to their customers, then it's not this trope. It would have to be a conspiracy unrelated to their product.