My first thought, upon seeing this cover, was that the lady in the dressing gown was offering the dude a rather easy choice: lead or booby. But even if the platinum crayon is not the sharpest in the box, that isn't a very likely scenario.
Upon consideration I decided that she must have just pulled the revolver out of her robe. But that doesn't really scan either. You can't tuck a heavy gun in a lightly-tied terry-cloth sash. And pulling it out rapidly would probably result in seriously chafed nipple, a hole in your robe... oh, and blowing your own foot off.
I must admit to never having read this Mike Hammer classic, is there really a scene like the one shown on the cover? I have to read the damn thing and find out. I don't remember anything fo the sort in the movie (1955) (which drops the patently unecessary comma in the title). But honestly it is the nightmare inducing final act that sticks in my mind rather than the rampantly noir femme-fatale-ing along the way.