A through-bore slip ring is one in which there is a cylindrical or stepped cylindrical space through its axis . Here is a picture of a through-bore pancake slip ring. A through bore-slip ring will not carry down its axis any other rotary joint, such as a fibre optic, microwave, pneumatic or a hydraulic.
In contrast, a flange-mounted slip ring can carry these additional rotary joints down its axis. The second picture is of a flange-mounted, cylindrical slip ring with an additional rotary joint down its axis.