This Stevens manufactured third variation mainspring with the upper forward section being flat is for the later Favorite single shot falling block rimfire rifle which not only mounts using the 2nd variation mainspring screw and 3rd variation hammer but lacks the lever spring screw which mounts on the inside forward belly of the receiver and can be seen externally. This mainspring is in very good condition exhibiting a grayish brown patina with light wear. (file photo)

Note: this part is commonly found on Stevens Favorite single shot rimfire rifles with 7th & 8th variation receivers.