Could you confirm that this is two items and that it fits a mighty-mo battery lawnmower 300 LI serial number 9671035-01 ?

Looking at the parts diagram for your model, I can confirm the will fit your machine. It's number 12 on the below picture. https://lsengineers.zendesk.com/attachments/token/gKzMkPlmL03xI2HPjYBqZmBGj/?name=FLYMO+MIGHTI-MO+300+LI.pdf