Some racks have M6 or M5 threaded sockets brazed or welded on to the rim of the rack.  These are almost certainly to mount a Nitto lamp holder.  This lamp holder comes with two sized bolts to mount to the rack, an M6 or M5, so you are covered either way.  It also comes with a nut, so you can thread the bolt thru, then use the nut to lock it out so it doesn't spin or back out of the rack.  Put some bee's wax on the threads before you install.  Nature's loctite, as Grant Petersen would say.  

We sell these in right or left configurations, in silver or cerakote colors.  Most folks want the right lamp holder, as it both works better with dynamo wiring AND illuminates the side of the road better when riding on the right side of the road.  But if you live in a place where you ride on the left side of the road, get the left mount.  Ideal for mounting a Schmidt Edelux or a Sinewave Beacon.  You'll also wanna get an 22mm M6 bolt to mount said light, washers, and a nut.  You can get that stuff separately or order it together with the mount below.  You'll get: light mount with rack mounting bolt options AND a stainless m6 bolt, 2 stainless m6 washers, and a stainless m6 nylock nut if you get the light mount with hardware below.