Certainly looks like a slim, some great information here: http://www.gilletteadjustable.com/slim.html . If it is E4 then it could be a real rarity, even if the date stamp was a factory error. Can you post a picture of the underside and the inside of the head once you have cleaned it up. I am assuming you are going to clean it.
Peek Polish shifts watermarks very easily. Has never taken more than 2 applications in my experienceThe more muck the more satisfying it is to clean (then remaining watermarks looking like easy peasy to fix can be a pain in the behind)