I think the dime is just so small smaller than a penny in fact no one can see it that good or wants to admit there may be one out there anything's possible.. there is a story in the coin magazine of some folks who sent in a 1916 dime and it turned out to be a proof and they weren't going to send it in cuz it was in bad shape now they are rich... so you never know
If you're going to send it in for certification because "you never know", then you'd be much better off spending the $50 on powerball tickets (in my opinion)
If Fred Weinberg says it's not an overdate, then that's pretty much good enough for me. Although people make mistakes, Fred has made his living dealing in rare coins and identifying errors. I take his opinion as more educated and better than mine.
We're not trying to be negative. We are being honest, straight-up, and helpful. You (the OP) have no idea how many people, think that what the find, or see on a coin, is unique, special, or valuable. I've seen this an uncountable amount of time in my 47 years as a full-time coin dealer. We're just trying to bring you back to Earth - or would you like us to simply agree with you because of what you think you see, even though we know it's not what you think? So, would you rather we cave in and agree with you to keep you happy (and apparently thinking you're now rich), or do we do you a better service by telling you what we have had to tell numerous people who post here with their new discoveries, new finds, unique coins, etc? I, like almost everyone on CT, would rather tell you the honest truth.....if you'd like to hear confirmation of what you think you have, you'll need to find other opinions, who aren't familiar with the minting process, circulation wear, die wear, damage/alterations, etc. In most cases, it's just damage - in the Merc. Dime case, the coin is fairly worn, and you're just seeing the normal effect of decades of circulation either moving metal, or corrosion or something else in a particular area that might seem to be a digit, a number, etc.
I agree with Fred 100%. I have been looking at every 1918 dime (and Lincoln cent ) I come across for almost fifty years. I even have two low grade dime "keepers" somewhere in my stuff and once saw a cent. Nevertheless, I am still not convinced either is an over date! There is reason to keep searching for FUN as 1918 was an "overdate" year. Unfortunately, until one is discovered in AU-Unc, THEY DON'T EXIST!