Looks like you were asking about the difference between them, rather than which one should you use. :) A letter that is "dated <some date>" means that <some date> is written or stamped onto the letter. The implication is that the letter was written on that date, but who knows? You can write any old date on a letter. I might write a letter today and put "Mar 14 2018" at the top but post it in a year's time. The letter is "dated Mar 14 2018". But that's not the date it was written. Now, consider the construct "<did something> on". The word "on" in this construct implies a date. "Delivered on Mar 14 2017" means that that's the date on which something was delivered. "Born on the 4th of July" means that the person was born on that date. A special case of "<did something> on" is "dated on". That really is the day that something was "dated". The letter I mentioned above was "dated on Mar 14 2017" (that's the date I "dated" the letter) but the letter is still "dated Mar 14 2018". The difference is subtle and people rarely care about "dated on", because they assume it was dated on the same date it is dated.

We say "letter dated 1 March" in formal letter writing

Hey Shawn, i'm not sure of a super official answer, but allow me to give you my best guess...lol So, I would say there isn't much difference at all with these two forms except that "dated on" would be more specific.... Honestly, "dated" is more natural at least where I am from. You know, now that I think about it....I'm not certain I've heard anyone use "dated on". So, in short, please use "dated" ex: The later dated 03/14/17. It's much more natural I think.