Mainsail boom track hardware

[duetto]

On my mainsail boom are two metal pieces that fit in the sail track (loose footed main). Attached to the metal pieces is webbing with a ring that hangs below the boom. What is it for? Thanks!

Note - Pictures were taken and uploaded in upright position. Not sure why they are sideways now.

[PerfectPirate]

This sort of rig is typically used to attach a boom vang to a boom with a non-fixed pivot. That is, a boom which is free to rotate a certain amount as well as swinging from side to side.

[psk125]

While some people may use such a contraption for a boom vang, a much more interesting function would be to hook a bosun's chair to it and slide crew out to the side to get the boat heeled over for going under bridges. Not only does it help enable the vessel to proceed, but dousing the crew keeps them cool on those hot summer ICW days. Make sure to heel to starboard, away from oncoming powerboats. You don't want their anchor rollers poking holes in your mainsail.