I built the 4" version and can tell you there is no coupler between the fins.
I also modified mine by moving the bulkhead forward ,so I have more room for my parachute and my own shockcord ,seems too cramped otherwise.
The rocket flies really nicely BTW ,my first flight was on a CTI 648J285 classic with 10 sec delay (1 or 2 seconds longer would have been ideal) nice and straight as an arrow.
Great choice for a kit and would be great for a dual deploy mod.
Paul t