Just a thought. It could be Australian, where they also used the sil shortening. There was a Stewart Dawson & Co trading in Melbourne, Sydney, Perth and Brisbane from 1885-1960 onwards. The mark illustarted in the book below is in a cartouche, but that's not to say they didn't have another.
Also, there is at least one Austin in Australia, and I don't know of one in India.
Miles
Ref: Australian Jewellers, Gold and Silversmtiths; Makers and Marks, p.235
Like you, the first name that crossed my mind when I saw 'S.D.& Co.', was Stewart Dawson, however, as they was such a large multi-national company I couldn't see them hand engraving their mark on a piece, rather than punching it, but who knows, maybe they did. Hand engraving the company's name appears to be quite a popular method of marking in mid 20th century India and I have seen several different examples of this type.
As for Austin, it possibly may have been the name of the person that sponsered the trophy.
Sadly, this trophy no longer exists. It was a victim of the melt, I was just fortunate enough to acquire the last images of the piece before it met its fate as I expressed interest in the mark.