the difference in the fighting purr call is you can make that call on any slate/glass/mouth call on your own or with someone helping you with both of you calling. I have used a fighting purr call for years on my slate call.
not to mention that fighting purr is a common sound you hear during all seasons. this thing is a bit out there, but just because you have not ever heard it before doesnt mean it dont happen (example being Hens that gobble)