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