An armourer would have made it flat due to pride in workmanship and not a half baked effort, the posts and springs are easily obtained the cross pins are there bit more difficult and as Peter has explained you can use a suitable sized nail cut down to put in the rear site axis pin hole.
I brought a spring and plunger for $10.00/AU from a seller on fleabay so they are not that expensive see if simon has a complete rear sight set up.....!