The fin on the Bell 212 was put on because one FAA test pilot considered that the machine had too much directional stability.
Hard to understand how anything could have too much directional stability, but there was some method to the thought process.
First of all, the FARs do not require much for helicopters in terms of directional stability or spiral stability.
Secondly, remember that the requirements have to be shown for all combinations of weight and CG.
So, at some combination of weight and CG, the directional stability was determined to be sufficiently strong that the helicopter exhibited a negative spiral mode (i.e. in a steady turn, the stick would have to held out of the turn to stop it rolling further into the turn).
The cause was thought to be the overly-strong tendency of the helicopter to line up with the relative airflow (i.e too much directional stability). The solution was the fin.
It was an interesting interpretation of the FARs.
Interesting to note that after the gentleman retired from the FAA, the fin was quietly removed...
At least that's the story as I heard it.