Namaste Sarabhanga,
Shrisukta has a line "tAm IshvarIm IM sharaNam prapadye" (citing from memory). And one Rigvedic passage mentions "chatvAra IM" which according to Srividya adepts refers to four kAmakalA-bIjas in kAdi-panchadashI.
Re designation of "IM" as pranava i know from oral tradition which goes from Sri Bhaskararaya (my SV Guru belongs to this line).
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