The affixes yat and khañ come after the word māsa in denoting 'age'.,
This debars ठञ् ॥ The word भूतः of the previous sutra should be read into this. Thus मासंभूतः = मास्यः or मासीनः 'a month old'.
Why do we say when denoting 'age'? Observe मासिकः ॥,