Unfortunately I can't help you in any way, but when you mentioned you replaced the stitch length knob with a lever I became interested. I didn't know it was possible. Almost all model 66 machines here are with the knob, the lever version turns up only now and then. Is it a lever from a different model or is it from a 66-16? I have read of after marked kits to give the old straight stitchers a backtack, but I have never actually seen one.
This link might be helpful, it's from a Vintage Singer Yahoo Group. It's directly to a pdf document.