For the old 12 twist is my guess also. In my experience the 115 Partition is one good deer bullet in a 25-06 if your rifle will shoot it accurately. Some rifles just don't like the Partition.
It looks like a typical .257 115 gr PT to me.
It shoots very well in the Remington 700 Classic 257 Roberts which has a 1:10" twist. Great bullet that's wicked on WT deer.
From Nosler
Designed to provide a shorter COAL in cartridges such as the 257 Roberts, these have a shorter ogive curve and a slightly more rounded profile than the standard Nosler 25 Cal 115gr Partition.