If you would look at the soundmatirx, you would be able to see exactly what prs file is used by what plane. I am not at home, and going purely from memory here, but I think that the P38s use the same motor prs file as the P39s, while the p40s have their own. Now, in your specific soundmod, the mod may use the same wavefiles between the two possibly, making the prs file a placeholder. The point is, a soundmodder could create a customised soundset for it if he wanted to, because it does use a (more or less) unique prs file, which can be easily identified with the soundmatrix.