Nothing wrong with the Argyll's, I like them and the last single clasp Modder that I bought was to a rather unlucky member of their 1st Battalion, recalled back the the colours and shot through both legs at Magersfontein.
Kind regards Frank
The main reason that single clasp Modder River QSAs are scarce is that many of the men involved had also fought at (and qualified for the clasp) Belmont as well.
As Frank has intimated there are single clasp Modder River QSAs to the Argylls out there, although they're far from commonplace, to Magersfontein casualties. The one below was awarded to 19 year old 6554 Pte. J. Dillon who sustained a gunshot wound to the back of the skull. Without wishing to be appear uncharitable this is most likely to have happened when the Highland Brigade infamously broke and ran during the battle.
Dillon was unsurprisingly invalided out of the army and his PIN 71 file shows he was unconscious for 3 days after the battle. The poor chap had an almost complete loss of power in his left arm and leg and was deaf in his right ear. He struggled along with his considerable disability, which severly restricted his capacity to earn a living, until his death in 1936 aged just 56.