The words 1. kusūla 2. kūpa 3. kumbha and 4. śālā have the acute accent (udātta) on the last syllable before the word bila,
Thus कुसू꣡लबिलम्, कूप꣡बिलम्, कुम्भ꣡बिलम्, शाला꣡बिलम् ॥ But not so here सर्पबिलम् ॥ Why do we say 'before बिल'? Observe कुसूलस्वामी ॥,