I found 3 cases labeled 6.5 MKS, 280 MKS, and 338 MKS. One source told me that the 338 was derived from a 375 Ruger, so I'm thinking that they are .375 Ruger Wildcats. Since the others have similar dimensions except for the mouth/caliber, I think they were all derived from the 375 Ruger. Rim...