Use blued slot 'blanking' screws, much as one does on rifles, e.g., like on the side of a Marlin lever where it is drilled & tapped for a receiver sight. Your screws are probably metric threads & could be hard to duplicate or match, but the blanking screws could easily be fabricated from the ones that presently hold on the front sling mount and re-colouerd/blued to closely match the bbl. finish.