Cant say for sure on the L frames, but the N frames I have that are factory drilled, are drilled completely thru the top strap.
If it were mine, I would have a gunsmith drill & tap two holes in the top strap, under the sight. Would mount a short Weaver type base that matched the profile of the top strap and the hole spacing, then mount the Fastfire to that using the picatinny / Weaver site base it comes with.
Due to the short mounting length of the Fastfire sight, you do not need more then two mounting holes drilled, or a full length base as you would need with a longer scope.
I have had mixed results when using the various "no gunsmithing" mounts on revolvers that have any kick to them.
Larry