okay after further looking looks like a flat tank from a harley from 1916-1920 thats what it looks like to me wish I could say for sure . where did you get it ??
1925 Royal Endfield - standard 350cc.......wild guess
Also looks like various Indian or Harley styles of the 20's but what is making it difficult are the two indented sections on the bottom as all of the pics I have seen show this area to be straight with no contour changes
If I could make out the top word I could find it but I know the bottom word is standard
Do not take the rust off, or try to polish it off yet. Some collectors don't like any patina to be removed, and if you do then make sure it's done professionally, well after you have fully researched it to make sure that it will not lose any of it's potential value.
let us know what said item is worth, also as other member stated do not I repeat do not remove the age of that item some guys just love to pay for it as is !!