Just a quick share, I have seen this pop up more than once, and wanted to show what I have used in the past. If you don't have access to the rear sight cover plates, which I haven't seen any for sale in a while now. Some have melted black crayons or whatever they can stick in the open holes after removing the rear sight assembly. I have used these automotive fasteners with excellent results. Simply find the head size diameter that suits you best, $.25 each typically at Ace Hardware, snip them to length with a pair of side cutters, and shove them in. This one below, the pin hole is slightly showing, but the threaded holes are covered. And once these are in, they are not coming out without some work. The flutes on the shafts snap into the threads, and it takes some work to get these back out.
Thanks for looking,