I don't think model 15s with stitch length lever ever came in the more elaborate decals? When was the lever introduced? I don't think it was until the 201-2 and 15-91 were around 1930. Not any of the pre 1960 machines ever had a knob for the feed dogs on the bed either (it was a setting with a screw underneat). I can hardly see the knob, if at all, but if it's there, it's almost sure it's a later reproduction or a Japanese clone redone with Singer decals.