Sud Atlantica Sociedad Anonima de Navegaçion : Operated pre WW1 with Lloyds 1904 indicating that it was operated by Miguel Mihanovich, it appears to have ceased operations around 1930.
From the plate shown in Znamierowski:
#376. Sud Atlantica Soc. Anon. de Nav. - Buenos Aires
Shipping lines: Buenos Aires - La Plata, Brazil Cargo and passenger steamboats: 8 Tonnage: ca 10,000 Regt. brutto.
Red flag with a white lozenge nearly touching the edges of the flag. A blue anchor in the middle with S and A, in blue, on either side of the anchor, respectively.
Talbot-Booth in his 1936 books shows the flag under Compañia Sud-Americana de Vapores in error.
In the 1930 Larousse Commercial house flag pages this flag — red with white diamond, its four points touching the flag’s edges; in the diamond, a blue upright anchor (without rope) accompanied by blue letters S and A, left and right of the stock respectively — is misidentified as that of Cia SudAmericana de Vapores, Valparaiso.