I originally figured GR directed the creation of unique patches for the Constellation and Exeter; after all, he once told the production team that each area (set) had to have its own unique sound. The Justman memo of 12/18/67 blows that theory out of the water though.... BTW, how positive are we that this memo is authentic?
I found the memo in the Roddenberry papers for the original series which were donated to UCLA and publicly accessible there. I have no reason to doubt its authenticity. Its condition matched the other documents in the collection and I doubt someone would have the motivation to fabricate a memo of this sort and include it with the collection -- and, if they could, I doubt they could sneak it in given reading room security.