Piston Ring 73.5mm Ring No 2.  72mm + 1.5mm (+60 Thou). for the 300cc Engine.  These rings are new old stock that have stored for many years and may not to a standard that you will except if that is the case return them immediately. They have been measured without the aid of specialist tools  and sized accordingly we maybe wrong. You can adjust the end gaps to suit the bore size but usually it be from one size oversize only. Look to get about 4 thou per inch of the cylinder bore end gap in your rings as you would normally when you fit piston rings, if you don’t know what I am talking about seek help 51/8