According to Wikipedia,a key party is a:
a form of swinger party, in which male partners place their car and house keys into a common bowl or bag on arriving. At the end of the evening the female partners would randomly select keys from the bowl and leave with that key's owner
The Urban Dictionary provides a similar definition.
Based on what I know about the swinger scene, it sounds completely out of line with their modus operandi, just fit for a flashy Hollywood treatment of it. (The concept is used in several movies, including The Ice Storm).
The source mentioned in Wikipedia is Social deviance. A substantive analysis, by Robert Bell. I cannot access the book and check how it justifies its conclusions, but based on the name alone I cannot take this source as unbiased - or even accept it as any source at all.
There is also some Snopes discussion.
I don't question that, maybe, this happened once or on a few occasions - in particular after Hollywood popularized the term much later.
My question is: Were key parties, as claimed, a somewhat common part of 60s and early 70s "social deviance?" Are there any sources that provide evidence more than claiming they exist, or existed?