Those will be the bore measurements at the muzzles compared to the true bore. 17.6 converts to .693" and 17.8 converts to .700" If the bores are the standard .729" for a 12 gauge, then you have choke constriction of 0.36 in one barrel and 0.29 in the other which is full and improved modified. Really the bores would have to be measured to know for sure if they started at .729 ,or if someone had the chokes altered then the measurements now would be wrong.
