The affix ṭac comes optionally after an avyayībhāva samāsa ending in a letter of jhay class (a mute letter).,
The झय् here means the pratyahara झय् i.e. all the Mute consonants. Thus उपसमिधम्, or उपसमित्, उपदृषदम् or उपदृषत् ॥,
ज्ञयः
Adhyāya 5 · Pāda 4 · Rule 111