Hello and thank You all for Your comments.
Of course You are right, it is a Garuda hilt.
Seems as if I did not read Coomaraswamy's Medieval Sinhalese Art well enough.
Maskell, thank You for the link.
Actually this threat is one of the sources why I believe this knife to be related to the Piha Kaetta.
Here is another threat to this toppic depicting Pihas with Garuda hilts.
http://www.vikingsword.com/ubb/Forum1/HTML/001998.html
Best regards, Helge