Thanks a lot for replying
I had already seen your quotation (M6 x 45 = 2 + M6 x 60 = 2) on the fin-bolt-chart. This quotation refers to 2013, while for 2012 there is no quotation for iSonics (and for 2011 there is no quotation for the 137L iSonic) so I supposed that the 2013 quotation should apply for my board too.
The reason Iím a bit confused is that I was given M6 x 100 to use for a simple (short) tuttle fin.
Actually after your reply I borrowed a simple tuttle fin from a friend and indeed the M6 x 100 screws fit fine on my board. But why do I need longer screws?