This an insert for the Lee Loadmaster carriage to hold a borroscope cam. Loadmasters are notorious for not properly installing primers. This allows you to run a borroscope and secure it in place with an M3 screw.
The borroscope I used was a cheap Amazon one approximately 5.5mm diameter. Picture quality is poor but and focal length is about 1.5 inches. You can still clearly make out if a primer is installed or damaged. A more expensive borroscope with smaller focal point would increase picture quality at less than inch.