Cheers EJ!
The board is a 2009 Tabou Freestyle 100...
Here's a pic of the fin:
Yup, in theory, it probably was a $170 fin... apart from the fact that it came with the board and was not fit for purpose... and no-one would buy it off me!
Now it works like a dream. Got a couple of one-handed spocks with it on it's first outing, and damn close to a grubby too... great day at Sandy!
Roll on tomorrow - I need that grubby...
[edit] ... just measured it - 16.5cm.