Even though the Fox B is a little better Stevens, it's still not a Purdey. These guns were assembled from assembly line parts & did not receive hand fitting. Surfaces were generally a little rough. May not have been much right of center to begin with & initial wear-in would have come quickly from simply opening & closing the bolt. Keep it lubed & it may surprise you how long it will go without further movement to the left.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra