The twelve point 5mm tool is a fine idea and I will certainly incorporate it at my next attempt at removal of the plug. As you may or may not know the actual allen plug head is broken from the threaded shaft which presents a new set of problems. My need to remove both pieces is paramount
so that I may drain and renew the A/T fluid for future maintenance. As I have the new replacement plug I can see that it is more than possible for the separation of the head from the threaded shaft. The PO could not have finessed the proper removal and consequently the broken head. My time my problem hopefully a shared resolution.

Thank-you for this moment in time