The Tallys are found

 

I hope you are well enough to come out today dear Lady Cowper and if  are at liberty I shall be mighty glad of your company to dine with me when you will find nobody but your friends and humble servants Mr and Mrs Clayton and Doctor Clarke .[1] We can’t dine until three o clock. It was my mistake concerning the tallys. I thought there wanted one, not minding that one of them was notched on both sides.

[1] Dr Samuel Clarke, (1675–1729), theologian and philosopher, mathematician and close colleague of Sir Isaac Newton.  He was famous as a distinguished scholar, Church of England cleric and was renowned as a scholar throughout Europe.  Politically he was a committed Whig which no doubt made him an ideal dinner companion for Sarah.