That number represents the measured dimension of the bbl. between the lands. They are usually in the range of 8.81, 8.82, 8.83. Since this I.D. is a little more than a tenth of a mm smaller than the 9mm slug, the lead or jacket gives way this much, causing the slug to ride the lands as it exits the bbl., establishing its spin. I think the middle number is actually a misstamped or deformed 8. Look more closely. On a refinished bbl. sometimes they have been worked down enough that the numbers start to disappear.
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