When I have a child entering Montessori at 5 and they can already count and know the letters, I still take them to the beginning of the Numeracy and Literacy curriculum and allow them to work with the materials following the correct sequence. Doing this has always brought about great success and achievements. Each Montessori materials has a direct aim which we know [ for example that the child practices counting to ten] and there are also Indirect aims and these are as important for the child to absorb. SO if the child can count to 100 I would still start with the sandpaper letters.
Here is Little-N working with the addition strip board.
The little Starfish in the picture is his new favourite toy. It came in a basket of shells for £1.50 from the local gift shop.