to install an old orthography Malayalam Unicode font which is required to read the posts below]
There are two different sets of graphemes used samvruthokaram. Usage 1: the sign of U + chandrakkala (visible virama) Usage 2: chandrakkala (visible virama) alone. These two practices co-exist today and had been like that for at least a centuary.Details
In Keralapaanineeyam - the formost grammar book of Malayalam - A. R. Rajaraja Varma criticizes those who argue for samvruthokaram to be written without the sign of U. This, in turn , tells us when Keralalpanineeyam was written (cir. 1896) the grapheme of samvruthokaram was an issue.
Frohnmeyer writes about this in University of Madras book printed in 1913. (thanks to Eric Muller)
Eventhough the following poem by N. N. Kakkad is printed (in 1988) in old orthography, the samvruthokaram is represented with chandrakkala (visible virama) alone.
When new orthograpy became common for printing in later years, the usage of chandrakkala alone for samvruthokaram became even more widespread. Vast majority of the printed material after ~1994 uses chandrakkala alone for samvruthokaram. As an example, see following poem by Balachandran chullikkadu (written in 1970; printed in 1996 by DC books):
Current status and brief history of grapheme(s) used for samvruthokaram is more or less accurately described by Dr. Scaria Zacharia in 1996 as a footnote description in Keralapanineeyam, centinneal edition from DC books:
Translation: "In the old Malayalam, there was no seperate grapheme to indicate samvruthokaram. It used to be written and printed as /paaTTa/(patt~) and /naaTa/(naaT~). In the second half of 19th centuary, Basel missionaries started to use chandrakkala(visibile virama) to indicate samvruthokaram (L.V.R. 1940: 329). Same words have been printed with and without samvruthokaram in Gundert's works. There is no unity in Malalayalam in the issue of grapheme for samvruthokaram. In southern Kerala, the sign of U and chandrakkala together is the grapheme for savruthokaram. However, in northern Kerala, just chandrakkala (visible virama) alone is enough."