After idam comes the affix thamu (tham) in the same sense of a kind or manner of being.,
This debars थाल्; thus : अनेन प्रकारेण = इत्थम् 5.3.4 'in this manner'. The उ in थमु protects the म् from becoming इत् (See 1.3.4).,
इदमस् थमुः
Adhyāya 5 · Pāda 3 · Rule 24