The rotary-magazine Johnson first appeared in 1937. As initially introduced, Johnson came in two models, the Model V having a drop-out Lee-type magazine with a Vertical feed, the Model R with the Rotary feed. The Japanese certainly would have known about the Johnson; it was offered commercially.
The 1941 Johnson, nearly identical to the 1937 rifles, was purchased for delivery to the Dutch forces in the East Indies, but delivery could not be made owing to the rapid Japanese occupation of that area. The rifles languished in storage until they were sequestered by the US Marine Corps, who had nothing better than Springfields, all production of the M-1 rifle bing contracted by the Army at that point. Shortly thereafter, the Japanese had all the experience they could ask for (from the wrong end, mind you) of the Johnson rotary magazine.
An interesting feature of the Johnson magazine was its fast loading WITH capability of being topped-up with a spare Special round. You loaded the Johnson with Springfield chargers, ran a round up the pipe and fired. Should you suddenly need a spotter round, an AP, Incendiary or Tracer, you just shoved that round into the feed gate, fired the round which was in the chamber..... and the rifle loaded your Special!
The Pederson rifle was rejected by the US military at the time of the adoption of the Garand, but the design was licensed for manufacture in England and rifles were made there and sold. There would have been no trouble for the Japanese to obtain a specimen or six.
The biggest problem the US faced in developing a semi-automatic rifle was the size, power and intensity of the .30-06 round. The '06 ran at 50 - 52000 psi chamber pressure, close to 10,000 pounds higher than most other rifles. Rifles which worked VERY well with lower pressures just could not handle the intensity of the '06 round. The .276 Pederson round ran at lower pressures and the rifle did very well; trying it at '06 pressures showed only that the delayed-blowback Pederson rifle and the '06 cartridge were incompatible in the extreme. The Pederson did, however, work very well at the lower pressures of the Japanese 7.7x58 round.
Combining the Pederson action with the Johnson magazine was uniquely Japanese and it was a darned good idea.
It would sure be nice to find a warehouse full of the things!
Yoo-hoo! NorINcooooooooh!