Yes, the engine itself was developed in 1923. I think think in this case, according to the article, the innovation was using this engine in a side to side configuration in a motorcycle instead of longitudinally which had been done before. Gave better handling stability and avoided the inherent over heating of the back cylinders. I think what the Japanese contributed to the motorcycling world was a transverse in-line four that used technology that someone else had innovated (like the Gilera), but made it cheaper and reliable for the masses. The Gilera was a racing bike. As we all know, there were many other manufactures with four (Brough Superior Austin Four), six and eight cylinder engines produced. Not sure how practical a six or eight cylinder motorcycle wold have been for the time. But I would think a four cylinder should have seemed reasonable. I suspect the cost along with the lack of frame , brake and tire technology was probably a factor in four cylinders not gaining traction with the public until the Japanese bikes came out. The first double overhead cam four valve per cylinder engine was introduced by Peugeot in a Grand Prix car in 1912..And that is still not so common. Interesting article.
