Hi Rick.
I think your best resource would be a Historical & Technical Society or some such. I vaguely recall that the NKPHTS had an article about horns in one of its quarterly magazines some years ago, though I may be hallucinating this. But if the WP and D&RGW have similar societies (I'm pretty sure they do), there might be resources there.
BUT . . . horns were one thing that changed early and often on locomotives, and the horn "as delivered" on any unit probably wasn't the horn on it 5 years later. The NKP GP7's were supposedly delivered with the Leslie A200 (the single-chime "duck" horn), but Tony Koester, who lived for a while right across from one of the NKP main lines, once told me that he recalled hearing multi-chime horns like the Nathan P3 or M5 or similar ones from Leslie. In theory, the horn you pick probably should match the horn detail on the model: if it's a single horn (true of a lot of the PA-1 models), then something like the Wabco or Nathan single chime would be appropriate. If the detail part shows 3 horns, then maybe a Nathan P3 or its equivalent Leslie. And if there are five of those things on the detail part, well, a Nathan M5 or similar would be called for.
So . . . I'm not sure that you can really get perfectly prototypical with this unless you have a photo of a specific unit that you are modeling, for a specific year, that shows the horn detail. And even then, unless the historical society has records from the prototype that specifies this information, you might not know if the horn is a Nathan or Leslie or custom job.
As a result of all this, I've gotten to the happy place of just picking a horn I like, and to heck with everything else!
John C.
