Red . . .
Congratulations on the Ruger KGP-141; I own two of them and they are GREAT handguns (reliable, durable, accurate, STRONG, concealable, and effective with the wide variety of .38 Special and .357 magnum loads.).
With regard to your question concerning the old, single box of Winchester-Western .357 magnum cartridges, they are probably okay. HOWEVER, WHY TAKE THE RISK? Whenever I see question like yours, I am compelled to ask are 50 rounds of .357 magnum FMJ (which you can replace easily and inexpensively through almost any reputable internet/mail-order ammunition outlet or at any gun show) worth potentially damaging a terrific, lifelong revolver that can bring you (and your children) generations of pleasure?