6.5X257 Roberts IS NOT the same as 6.5X57. The Roberts case has a 20*45' shoulder and a 1.912" datum measuremebt, as does the 6.5X257 Roberts, whereas the 6.5X57 has a 18*55' shoulder and a 1.941" datum measurement. They are not the same round!!
6.5X257 Roberts is a wildcat round designed in the 1950s, and 6.5X57 Mauser is one of the oldest rounds chambered by anyone (having been designed in 1895). Both are based on a necked down 7X57 case, but they are not identical!