Even if the bezel size of the RRT21 is idedantical, the threads to screw it in could be different. Getting JetBeam spares is extremely difficult - I have seen posts of people who couldn't even get a tail switch.
You are better off keeping the SRT-6 bezel or simply removing if for you is a real issue.
It's my opinion but the window breaker is marketing fluff. Breaking windows is not that easy and it obviously depends on the glass. I could be wrong but looking at the SRT-6 bezel the crenulation doesn't seem to be much different than that on many other flashlights and not very aggressive either. For breaking windows, there are window-breakers which are specifically designed for that purpose and probably work much better than the stainless steel bezel of a flashlight.
Seriously, one of these days I'm going to set up a pressure cookers company, crenulate the bottom and then sell it as the ultimate cooking pot for home defense and fire escape: I want to see how much it increases sales!