The affix ṭhak comes in the sense of 'this is whose Art' after a word denoting Art in the Nominative 1st-Case in construction.,
Thus मृदङ्गवादनं शिल्पमस्य = मार्दङ्गिकः \a drummer\, \an expert in playing on mridanga drum\. So also पाणविकः, वैणिकः ॥,
शिल्पम्
Adhyāya 4 · Pāda 4 · Rule 55