I used a 2nd Generation (Model 639) ambi safety on the Model 915 slide I used for my Franken-Smith project, but in retrospect, one of the 3rd Gen units would have fit just as well and eliminated that pesky screw (that constantly needs retightening.) If you want to stick with blue, the primo solution would be one from a 3904 or 5904, but since my build was based on a 639 lower, the stainless unit was what I wanted.
Froggie