The Bertha is an outgrowth of the Ranger. The Ranger was, as noted, a payload rocket, and it had the cluster mount as a way to lift modest payloads when only 18mm motors were available. The design was then simplified a little later to create the Big Bertha. I strongly suspect that although the fins may be marginally different, a lot of that was probably the result of cutting variation from model to model, since the early versions required the builder trace out and cut out their fins on sheet stock. I'd bet that they probably were intended to be the same, but just evolved through iteration. Another note along those lines, the Ranger (and early Bertha) used a BNC-60L nose cone. That was changed, and the modern Bertha uses a PNC-60M. Purists would have to have the original cone, not only because its balsa (which to me is inconsequential after its painted), but there is a visual difference in the length and shape. The -60L is about an inch longer, which is a lot when the cone is only 3.5" exposed.
So, if you wanted to clone the Ranger from a Bertha reasonably accurately, you need the cluster mount, a structure change to allow the payload compartment, and a replacement nose cone. As I recall, eRockets has Semroc BNC-60L cones, but also have a few PNC-60L molded cones as well.