The 15^th and 16^th letters in Sanskrit alphabet are “aṁ” (anusvāra) and “aḥ” (visarga).
In ṣaṭcakra japa we add “ṁ” (anusvāra) to each one of the letters of the sanskrit alphabet.

When we add anusvāra to these two last letters, they become “aḥ” and “aḥṁ”. To make it easier to understand, if we look at these letters in Devanagari, in the first case one "dot" becomes "two" and in the second case "two dots" become "three.

1. aṁ (अं) + ṁ (•) = aḥ (अः); 
2. aḥ (अः) + ṁ (•) = aḥṁ (अःं)