Whoops, typo Deejay. 21 May 1968, at Plant 3 (Rolla, Missouri). That makes it even more likely the engine was built into a mower before the end of 1968. Do you know what month manufacturing of the SB45 began?
Also, with regard to removing that cut-off spurious shaft from the mower's engine sprocket, it appears to have been screwed onto the shaft as a replacement for the original sprocket nut, which is the same part as the drum sprocket nut, the cutter sprocket nut, and the chain adjuster nut. If you verify that any one of those others has a right-hand thread, you can then unscrew the piece of bar from the engine shaft, and use it as a thread sample to obtain another nut. To keep the engine shaft from rotating when you unscrew the piece of bar, you will need some kind of pipe wrench, but I suggest you wrap a piece of leather around the engine shaft so the pipe wrench doesn't chew it up and make it unsightly on your collector's-grade mower.
