Thank you, Prof. Ryan, Prof. Deshpande, and Prof. Cardona for your responses. I had been travelling and so could not respond earlier.
Here are some quick comments.
1) ‘śuṣrūṣamāṇānām’ is certainly a digitization error. The reading in Bhāgavata Purāṇa 1.1.13 is ‘śuśrūṣamāṇānām’ with the palatal ‘ś’. The ātmanepada is from ‘jñāśrusmṛdṛśāṃ sanaḥ’ (A 1.3.57).
2) The RV example of ‘ajuṣran’ is a non-Paninian form, the Paninian form being ‘ajuṣanta’ from the root ‘juṣī prītisevanayoḥ’ (DP 1288). The two non-Paninian features are the ‘ruṭ’ augment and the use of ‘parasmaipada’. Sāyaṇācārya explains the ‘parasmaipada’ by ‘vyatyaya’ (which would refer to ‘vyatyayo bahulam’, A 3.1.85), and the ‘ruṭ’ by ‘bahulaṃ chandasi’ (which would specifically be the rule 7.1.8). I have attached the snaps of the mantra and the commentary (Vaidic Samshodhan Mandal, 1935, Volume I).
3) I have not had the opportunity to cross-check the cited examples against published editions the Śatapatha Brāhmaṇa, hence I cannot comment on them now.
4) In a word boundary, a final ‘ṣ’ cannot occur in the Paninian system due to ‘jhalāṃ jaśo'nte’ (A 8.2.39) which will change a terminal ‘ṣ’ to a ‘ḍ’
Most likely, the examples of ‘ṣr’ do not occur in the Paninian system.
My interest to trace an attested form was partly because I wanted to see how the conjunct was printed in old books. It appears that the printing press where the Vaidic Samshodhan Mandal book was printed did not have a printing block for the glyph ‘ṣr’ (quite possible given the conjunct is so rarely attested). The ‘r’ hook was put below the glyph for ‘ṣ’, which is unexpected: given the similarities between the glyphs for ‘p’ and ‘ṣ’, one would expect the glyph for ‘ṣr’ to be similar to that of ‘pr’, with a slanting diagonal line as seen in modern fonts. Perhaps manuscripts of RV would need to be seen to ascertain how the glyph was actually written by scribes before the use of printing presses in India.
Thanks, Nityanand