Many 257 Roberts cases stamped +P.
I found the Winchester variety to be slightly heavier than the standard case, requiring a small drop in max loads.
Problem with the Roberts is most earlier factory loads are quite anaemic.
You often find loading data reflects the older standards [120 grain at 2700-2750 fps]
In reality, I have yet to see a Roberts with a 24" barrel that will not drive this weight bullet to 3000 fps safely.
The +P factory loadings are considerably faster than the standard loads.
Regards, Eagleye.