Mike, most of Numrich's parts are for Featherweight frames. Now I can't be sure as I have never measured or put one on top of the other to see if one was longer, but I do know that I have gotten mainsprings for Numrich and the short leg has to be ground to fit just right or it is not going to work right. I do believe this was for a FW frame gun.
I do believe it shows a picture of this in Disassembly-Reassembly.