Dean, this is just my guess, but I believe that tapering the area of the barrel under the rear sight base and matching the taper on the inside of the sight base is a waste of engineering and labor. I feel that if anyone were to take a rear sight base off of a genuine Luger carbine barrel, they would find that the barrel is a cylinder shape and so is the inside of the rear sight barrel band (in a similar manner to a K98 rifle barrel/rear sight) strictly from an ease of manufacturing standpoint.
If anyone might know the answer to your question, it might be Art Buchanan... track down one of his posts "Art" and send him a Private Message... which should trigger an email to him, as I recall he has that option enabled on his profile...
...and PLEASE dont forget to take photos of the setup and process as you work!
