The M28 did not come with factory front sight inserts, so yours is an aftermarket job.
There are various front sight kits available from Brownell's, but it is just as easy to make your own.
I use 10 minute epoxy from the hardware store. Mix in one small drop of Testor's model paint in your color of choice: dayglow orange, yellow, red, fluorescent green, etc and mix well.
Prepare your front sight blade ahead of time using masking tape to make a "dam" along the sight blade. Pour in the colored epoxy mix, and dab with a toothpick to get it into the voids. Wait until the pour is a solid gel, but not quite hard. Use a sharp razor blade to trim the sides and top to shape and allow to harden before use. For a more permanent repair, I use a center punch and dimple the bottom of the insert cut. The epoxy will grab better. I've done several sights this way, and none have come out.