To give some background for the others, the main difference between Tamil Brahmi and Asokan Brahmi is that whereas in Asokan Brahmi a consonant sign has an inherent "a" sound, in Tamil Brahmi it does not, and a matra must be added to produce an "a". If you look at the last two symbols in the photograph in the Hindu's article, you'll see they're read as "ka" and "l". The little bar on top of the cross adds the "a" to the "k". The "l", lacking the bar, is a pure consonant. In Asokan Brahmi, these symbols would have represented "kaala" rather than "kal".