Here are a couple Peter;
http://www.vikingsword.com/vb/showpo...64&postcount=4
http://www.vikingsword.com/vb/showpo...04&postcount=9
Several others both single and of the paired types on the second link are also known with filed backs...
Personally, I believe both of these types are from the Laos regions inland from Hanoi...based on a Dah I have with fine provenance in identical dress to the pair in the first link...but there are years of study still to be done for both.
Gavin