The a in an is not elided when the bha aṅga (stem) ends in van or man with a consonant preceding v or m,
Thus पर्वणा, पर्वण, अथर्वणा, अथर्वणे, चर्मणा and चर्मणे ॥ Why do we say 'preceded by a consonant'? Observe प्रतिदीव्ना, प्रतिदीव्ने, साम्ना, साम्ने ॥ Why do we say 'stems in वन् and मन्'? Observe तक्ष्णा, and तक्ष्णे ॥,