RE: Larger Fin for my F155
Everything you are telling me supports the idea that your F-155 does indeed have a Deep Tuttle (DTT) fin box.
If the back of the fin box measures about 2" and then the box straightens out, and goes up to the top of the box, then you almost for sure have a DTT box.
The rear dimension on a DTT bos is 2.038" measured on the 10 deg. rear taper angle.
The front dimension (on the 10 deg. front taper angle) should be 2.698".
If the front taper also blends into a vertical cavity, then a DTT fin should fit in nicely with a lttile fitting.
If you have a std. Tuttle finb box, the front taper would be only about 2.00" deep (measured perpendicular to the bottom of the board) and 1.25 " deep at the rear (perpendicular to the bottom also).
If you know anyone who has a deep Tuttle fin, just try it in your fin box.
If it goes in to a solid stop with the root still sticking up about 0.700" (18,00 mm) then you have a std. Tuttle box.
If the test fin goes in nearly all the way, you definitely have a DTT box and you can buy any DTT fin and make it fit.
Hope this helps,