The BNC-20L has always been an oddity. My original Mini Bertha and a BNC-20L I got off eBay years later looked very similar and nothing like a downscaled BNC-60L that would be expected. I assumed that it was made from a "recycled" stone that had previously cut some other BT-20-based nose cone and had been dressed a few times too many. The lower section is almost parallel to the tube for over half of the length.
The shape of the BNC-20L on Mini Berthas in photos in the catalogs evolved over the years as well. Like many of the Estes nose cones, we had to settle on one shape for a range of shapes to choose from. In most cases, we chose the shape to match an actual nose cone we could scan, measure, etc.
Another example is the BNC-30D that was probably the first balsa nose cone that Vern made, since it was used on the K-1 Scout. Measurements show the exposed length to be 1.38". The earliest catalogs showed a length of 1.5" long. An early one I had was 1.5" long and matched the catalog dimensions. In this case, we released the BNC-30DE (Early version) as well as the BNC-30D (later version). I almost released a BNC-20L version that was a downscale of the BNC-60L. I did release the BC-818L, which is a downscale of the BNC-60L.
But... the BNC-60L is also a moving target. Comparing photos, catalog & MRN pictures, and actual nose cones, there are probably at least four unique shapes over the years. We have changed OUR BNC-60L twice to match what most people perceive to be the “correct” shape.
I have not seen as many differences in Centuri nose cones as I have in the Estes nose cones over the years. I am not sure if this is the manufacturing processes or some other reason. Maybe the sheer volume that Estes produced was much greater. I know that Estes did have to redress the stones many times and that is an art, rather than a science. The new nose cones after a resurfacing will not be identical. Even nose cones cut over a period of time from a stone change shape as the stone wears. In the early days of Semroc, we resurfaced our stones several times as they wore. Since all our stones were ogive or conical, we used a diamond bit on a circular arc (ogive) or straight line (conical) to ensure the stone stayed the same after redressing. If we had done elliptical or parabolic nose cones, it would have been much more difficult to keep the stones the same.
Using computer controlled grinders, rather than fixed stones, allows for manufacturing large quantities of identical nose cones now. Picking which shape to make large quantities of is the much bigger problem.
We now produce over 600 unique balsa parts (not all online) and should be able to produce the exact shaped parts years from now.