Don't forget the 12" disco versions of songs in the 70's and 80's! The disco version was lengthened and mixed with a much heavier bass line. The grooves on a 12" single were spaced further apart to accommodate this.
It wasn't even just disco, that was a general fashion back then.
I remember Trevor Horn writing in his book, labels asked him for 12" mixes of singles he produced in the 80's. And it was mostly new wave and pop stuff.
I guess the purpose was the same though, to produce a more danceable version.
