Actually it is quite easy to tell what kind of torpedo it is. Its propeller, tailfin, diameter and any paint. All countries used their own torpedo designs and all were different. Different lengths, different weight.
It was taken to Scapa pier 9 before being towed off for destruction so I am sure the "Experts" got a look at it before deciding that boom at sea is better than boom on shore.
Also if you look at
www.u47.org you can see that a few German torpedo's Prien fired were misses and duds and I can assure you that those were the only German WWII torps fired in scapa Flow