could be for attaching a spotting device, something that shoots tracer rounds. or a subcaliber device used for training to sight the weapon in. many years ago.....20 or more years back, some civilain whiz kid, who had more education then common sense, at the Logistics compound at Ft. Lewis, came up with this hookey platform that was mounted on the VULCAN barrels, so a M60 MG could be attached, with a solonid attached to the trigger that connected to the gun motor, so that when you pressed the control panel trigger, it activated the solonoid on the M60 and fired the ammo from it.........it was suppose to let gunners practice areial firing on Ft. Lewis, without having to go to YAKIMA TRAINING CENTER to fire 20mm rounds........the concept seemed sound, but in practical application it sucked......when you elevated the cannon to track the aerial target, a giant RC styroform plane that looked like a MIG-27, the rounds didn't feed right into the M60, and it constantly jammed.