I don't believe that anyone owns the rights. As you note, contrary to popular opinion, CCSB does not have a claim to the Fish. I don't know what drawings MIT may have. The challenge you may run into is that the Fish was based on the same model as the 12 1/2. If I'm not mistaken, they used the same offset booklet as well, scaling up the figures as appropriate when building the molds. That offset booklet, I would presume, is covered by the CCSB agreement and is most likely in their possession.