What is this thing? Comes from a batch of Cox parts

I recently aquired someones collection of Cox parts and oddments. Among them a handful of .15 parts as shown in the photo. I know nothing at all about Cox .15 engines.



I'm taking a guess that the black hexagonal thing may have something to do with 1/4" threaded prop shafts based on it's measurements but I'm not sure. It is some sort of metal and is 1/2" exactly measured across the flats, 0.9" long and the internal bore appears to be 1/4" threaded but I can't be certain. Does anyone know what it is and does it belong with these .15 parts?



I also have a .15 needle and spring as shown, the needle is hollow and the spring is smaller in diamater than the one installed on the medallion .15 carb / venturi also shown in the photo. The needle installed on the carb is not hollow. Are all .15 needles the same or is the loose, hollow one, intended for a specific engine?



Thanks for any help,
Simon

Simon , the needle is hollow on the .15 Medallion , don't know if the solid one is original or not , does it screw into the carb. body ? and the coupler could have been used for removing and installation of the crankshaft - the stud if it is the thread size of the prop screw ? Will the prop screw go into the threads of the coupler ? See this http://coxengines.ca/assembly-tool-for-cox-.049-engines.html  Eric