Bryan: Hunter Arms did not introduce the 20g until 1907, and that SN would be for 1928.
Look through "Shotguns" and the FAQs on the Home Page for lots of information.
Please be aware that the chambers were originally 2 1/2" and you should not use 2 3/4" 20g shells.
Let us know where you are and we might be able to recommend a qualified gunsmith to check & clean the internals, and replace the firing pins. He could confirm the chamber length and chokes also.