letters that "complete the rectangle" in Indic scripts

I have been told that Devanagari contains letters (or a letter) that
were invented merely to complete the rectangular C-V table; not sure to
what extent they (or it) were used subsequently.

Wiki
     http://en.wikipedia.org/wiki/Devanagari
tells me about the letter ॡ (signifying "ḹ", I assume this means a
syllabic long "l"). Are there other examples? What about other Indic
scripts?

Stephan
Received on Tue Sep 17 2013 - 17:22:11 CDT