OK....just throwin this out there as an interesting feature of this Merc. Look at pictures (sorry for pic overload). A 'D' Mint, but along side of it, looks like an 'S' mint mark impression.
Yes...crazy toning on this lady, but in-hand it is strangely attractive. Pics no help in that department. Though most will think it's Fugly. But here for MM question, not Merc beauty contest. Lol. Nice Bonnet Die crack noted.
My question is, is this really there as a result of maybe a Die that was sent from the San Francisco Mint, to Denver Mint, and reworked to be a Denver Mint Die? Seems possible. And the 'S' mint mark did not get completely removed and we see what we see here?
But, I would think it would be on many other 1940-D coins as well. Maybe it is, but the Tone brought it out on this one?
Anyway....just an interesting find and thought some Mint Mark savvy folks could pontificate on whether it is anything at all. Probably not, but worth the look I think. Coins always have crazy mysteries from time to time. Maybe this is one of them. Maybe I need to see someone? :-) Check pictures and see what you think.