Could you tell me what the inner diameter of a ring mount mean? Does it mean the smallest diameter of the cylindrical components I can mount, or the biggest diameter?

The inner diameter is the circular or elliptical “short axis” diameter for which our Ring Mounts are optimized. Specifically, for #54-339 this range is 25.1mm to 30mm, and for #38-513 is 28.5mm to 30mm.