FWIW, those "holes for a tang sight" are where the Williams FP mounts on my 1952 Model 99-EG.
Mine's great - don't forget that a higher than issue front sight will be needed in order to achieve zero, a great time to change sight bead type to Ivory, Gold or Fiber-Optic if wanted.
While you're ordering, either order a slot filler blank for that vacant rear sight location, or file one up out of the dovetail section of an old longleaf open/iron rear sight leaf.
.
.