Frederick: I am probably missing more than just a key!
So, after some research it turns out that there was a brass barrel lock with a key for this Al-Ko Kober coupler.
I have even found the part number for this (203216)... just can't find anyone in North America that will sell me one! I'll bet Lex has seen these before!
When the brass piece was left in the coupler, the coupler is disabled. Insert key into brass piece to remove. Seems pretty simple.