I haven't updated this thread for quite a while.
As possible origins within southern India, there have been suggestions to a link to the late Vijayanagar empire and/or possibly local kingdoms developing from it (including more specifically Madurai in today's Tamil Nadu).
There also have been thoughts towards the NW frontier area of a greater "Indian" sphere of influence as well as central Asia (cp. this thread:
http://www.vikingsword.com/vb/showthread.php?t=21429).
More insights and examples more than welcome - this enigma certainly hasn't been fully solved yet!
Regards,
Kai