Hello Ian,
We all know that descriptions from auction houses could be very wrong. I've handled some of these daggers before and never had the feeling to hold an antique example in my hands. Look also here:
http://www.vikingsword.com/lew/w1147/w1147.html
I have a very similar example like the one from Lew, it's of very good workmanship but it's for sure not antique.
Regards,
Detlef