The 66 was introduced in 1900 I think, and yours could very well be 1915 by the looks of it. I think they stopped making the more elaborate decals some time in the 1920s. There serial number is usually embossed in the metal, when dirt and rust come off there's often enough left to identify the numbers. Be carefull, when scraping off the rust; this part once had a layer of transparent shellac too. If you have a tub of Quick-Glo it does a good job on this type of rust, it's gritty enough to scrub it off, at least partially.
I hope you will post pictures after cleaning and oiling too :- )