On someone swinging the ball before Bart, who knows. There were those who might have bowled leg breaks like Barnes and managed to bowl one that swung in and then moved away. It would be noticed only if it well directed as well as on a length. In anyevent it would have been an accident rather than design.
Someone mentioned Warne's ball to Gatting as a sign that he could bowl the same delivery as barnes did, well I wonder why we call it the "Ball of the Century" in that case - if it was so common place in Warney's armoury. Warne himself, in the interviews says, "it just happened" just one of those things.
I think there is a reason for no swing bowlers and that lies in the history of bowling.
We will discuss this in detail in the history of cricket thread but here I would say that, probably, since spin and break off the wicket was what bowlers used most to deceive batsman as far as lateral movement is concerned (length and line taken as given) all budding bowlers started with grips that gave them maximum chance of achieving the maximum rotation on the ball to get the maximum lateral movement after pitching. It is difficult to imagine a possibility when it has never ever existed. Its possible that some bowler, or even a kid in a park, bowled a ball that moved in the air and no one took notice. What counts is for someone to do it, even accidentally, and then perhps repeat it and then realise what was happening, then try and find what was happening, try to and eventually master it in the manner Bart King did. There is no record that anyone before ever did it.
Its quite like with Bosanquet and his tennis ball. He just happened to 'notice' a phenomenon and worked on it. It may have happened before him sure its possible, but if no one did anything about it - it doesn't count