I found this vintage toy at an estate sale a number of years ago, and it is one of my most favorite toy finds.
At 5, C is interested in numbers and counting and we have been doing some basic addition and subtraction problems together. The old plastic Add-A-Count is such a great visual for him. It has each number 1 -9, plus a couple of additional low numbers. Stack a couple of numbers together on one side and their total on the other and the arrow will point to Right. It even works with larger sums like 9/4 on one side and 6/5/2 on the other. This toy should be reproduced and sitting on toy store shelves, in my ever so humble/jumbled opinion.