Definitely do not put any marker or anything on that box, the value for me would go down further and many hangtab and sticker seal collectors would not be happy. I had a black box clu clu land years ago and it had all marker touch up on it from the previous owner and any time I held the box it just jumped out so I found an upgrade and sold it off.
Being a sticker seal is a fantastic deal for that price, correct manual aren’t that hard to find, I don’t think I have a spare though. I would love an extra sticker seal but I doubt I have any trade bait for you. Nice job on the pickup though.