I had the same issue, and it must have been common at one time. An RCBS #11 shell holder solved the problem. Found that info in an old Speer manual. All my current 257 and 7x57 brass fits in the #3.
+1 If I remember right, some of the old shell holder charts gave two different numbers for the 257
I've found this too. Just don't mix brands of brass in the same run. I necked some 6mm Remington up to 257 when I couldn't find 257 Roberts brass. It takes a #11. I only use it for 75 grain Sierras so it's always separate.
I've also found instances where a Lee holder didn't work with some 7.62 Nato, but an RCBS did. Just different manufacturers dimensions stacking up.
