The affix ṣac comes after a bahuvrīhi samāsa ending in the word aṅguli when the compound means 'a piece of wood'.,
Thus द्व्यंगुलंदारु, त्र्यंगुलंदारु, so also पंचांगुलं ॥ This is the name of a wooden spoon or fork of the figure of a finger by which barley &c are scattered.
Why do we say \in a Bahuvrihi\? Observe द्व्यंगुलयष्टिः = द्वे अंगुली प्रमाणमस्या ॥ It is a Tatpurusha compound formed by 5.4.86, by adding अच् as samasanta affix, the anguli meaning here size and not figure. Why do we say \when meaning a wood\ ? Observe पंचांगुलिर्हस्तः ॥,