When did "Happy ending" get used as a euphemism?

OED has the earliest citation of the slang term happy ending from 1999:

‘We are seeing more and more women accompanying their men in here,’ she says of her parlour, which provides ‘massage only’ but with a guaranteed ‘happy ending’.

_{Weekend Austral. (Nexis) 14 Aug. (Review section) 8}

The following analysis by Grammarphobia does not cite when the expression was first used euphemistically, but it provides interesting facts:

Happy ending:

• Both the noun “end” and the gerund “ending” mean, among other things, a conclusion. So “happy end” and “happy ending” would seem to mean the same thing.

• Although both are technical correct, “happy ending” is the idiomatic phrase (the one used naturally by a native speaker) when referring to the happy conclusion of a novel, play, movie, and so on.

• The earliest example of the expression in the Oxford English Dictionary is from Memorable Conceits, a 1602 translation of a book by the French writer and printer Gilles Corrozet:

  • “A good entrie or beginning is not all, without it haue a happie ending.” (In the original French, “happie ending” is heureuse issue.)

• And here’s a citation from a May 10, 1748, letter by Samuel Richardson in which he discusses a scene from his recently published novel Clarissa:

  • “The greater Vulgar, as well as the less, had rather it had had what they call, an Happy Ending.”

• The OED defines “happy ending” as “an ending in a novel, play, etc., in which the plot achieves a happy resolution (esp. by marriage, continued good health, etc.), of a type sometimes regarded as trite or conventional.”

According to the source, the OED does not provide usage examples with the euphemistic meaning, but @ermanem has actually cited one:

• The dictionary adds that in the US the phrase is also used for “an orgasm, esp. one experienced by a man after sexual stimulation given after (or during) a massage.”

• The OED doesn’t have an example of this usage, but the comedian Jim Norton uses the phrase in the sexual sense in the title of his 2007 book, Happy Endings: The Tales of a Meaty-Breasted Zilch. The cover shows him lying on a massage table.