The affix ṣṭran is used after the verb pū 'to purify' with the sense of instrument when denoting a member of the body of a plough or of a hog.,
The verb पू includes both पूङ् and पूञ् as पोत्रम् 'a plough-share'; पोत्रमे 'the snout of a hog'.,
हल-सूकरयोः पुवः
Adhyāya 3 · Pāda 2 · Rule 183