There is the single substitution of the first vowel, when a simple vowel is followed by the a of the case ending am,
The word अकः is understood here. Thus वृक्ष + अम् = वृक्षम् (VII. I. 24) प्लक्षम्, अग्नि + अम् = अग्निम्, वायुम् ॥ The word पूर्व 'antecedent, first' in the sutra shows that the first vowel itself is substituted and not any of its homogenous vowels. Otherwise in कुमारी + अम् = कुमारीम् the ई would have been of three matras or measures, as it comes in the room of ई + अ, the aggregate matras of which are three.
In the Vedas, there is option, as शमीं or शम्यं, गौरीं or गौर्यम् ॥ अम् is the ending of the Accusative singular of all genders and also of the nominative singular in the Neuter in certain cases. (See 7.1.23, 7.1.24 and 7.1.28.,