Dear Harry Spier,
according to my limited knowledge, Vājasaneyins pronounce(d) Anunāsika before semi-vowels, sibilants and h as "ṅg", represented in writing by a special sign (as in your edition): śataṃ śṛṇuyāma → "śataṅg
śṛṇuyāma".
Quite similar to how Germans - in the majority of cases - pronounce French nasalizations, e.g. "Kartong" for "carton"
However, the pronunciation when reciting Saṃhitā texts replaces the articulation of Anusvāra with a spoken "guṃ". So "tā guṃ haitām ..." for tāṃ haitām ..."
Your symbol represents an Anusvāra sign in a particular phonetic environment.
Vedicists will know better.
Best,
WS