We may have had this discussion before. 266K is mid to late 1924. That means that there were 10 years worth of 250-3000 and several years of 300 Sav. made before Savage made the change at 266K.
Note that the letter to Benenson says they won't convert other calibers to .250 or .300. Does that mean the pre-266K rifles in other calibers were treated differently than the .250 and .300?
