Your understanding is correct, Pat. For some reason, they all came with 1:12 twists. However, they are quite accurate with heavy round nose bullets.
I have owned at least half a dozen of these rifles over the years, and all of them did well with spitzers up to 150 gr in weight. Sold one to a good friend of mine, and it will shoot 160 Partitions close to an inch, but it is the only one I ever had that would.
If the S&B and PPU loads with 173-grain bullets are round nose I'll bet they will shoot that ammo just fine.
Ted