letters that "complete the rectangle" in Indic scripts

[Stephan Stiller]

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