I believe..... and bearing in mind that I also believe in the Tooth Fairy, Santa Claus, and Scotland's chances of winning the World Cup you can weight this accordingly.... that the term Self Loading Freight originated in the hilariously funny* "Uncle Roger" feature at the back of each issue of Flight International magazine.
At least, that's where I've seen it in most use.
£6
* About as funny as a week in jail, that is.... or intimate body piercing with blunt instruments....