This is a NOS, new old stock, Nelson 0491 muffler.  These were used on the 11 and 12hp air cooled Briggs and Stratton engines and may have been used on others as well.  It is in new condition and has never been used on an engine.  It has a 1 1/8 inch outside diameter inlet pipe and mounts with two 11/32 holes 1 7/8 inches apart on center.  It has some scratches from handling over the years but is a brand new muffler that is ready to be used. I can be reached at 262 573 1931 if you'd like to talk about it.  The pictures tell the story.  Good Luck.