I bought a sample pack of 50 blades from ebay as I was new to D E shaving, so I could find my go to blade in this sample pack, there was Wilkinson sword, shark ss, super-max ss, perma-sharp, derby extra ss, astra ss, astra sp, bluebird ss and prime platinum. I have enjoyed shaving with the Wilkinson sword, super-max, perma-sharp, astra ss and sp, and prime platinum. But I haven't enjoyed using the derby extra or the shark ss. Then I tried the bluebird ss and had some great bbs shaves, I looked on the Internet about them and some people are saying that they are re-branded derby's. Is this true? And if so how can two blades be so different if they are the same blade?