Couple of things-
The original design had no retention mechanism at all, just the stick stuck down in the socket. I believe there was a landing mishap where the right seater pulled the stick out, and that resulted in the Service Bulletin.
Second point. Even on Type Certificated aircraft, the FAA have no say in whether or not you comply with service bulletins. Even the ones that say "mandatory" really aren't unless they have been incorporated into and AD.
I say this because of the post a couple up from mine; Yeah, during an investigation they may want to see that the service bulletins are complied with, and yeah, the lack of compliance might be causal and open you up to some liability, but in this case, they aren't the boss of you...