This is a Johnson antique outboard ignition coil. Johnson part number is #375189. This will fit all the 2.5hp model HD20 and HD25 as well as the 5hp models TS20,TD20,and TN25-26.  There were over 100000 of these really good engines and many have survived.  New coils are no longer available and good used ones are getting hard to find.   Look at the photos and email with questions.  Thanks for looking.