That is not even close to confirmation enough, especially when they are auctioneers, not error experts. Get it seen by an error expert before you just assume they are correct.
Have you taken a wet toothpick and tried to wipe it off? If it's something real, it won't wipe off. If it's just some crud stuck on there, it will. Have you tried that? If I'm not mistaken, someone has already stated that it's just stuck on crud that happens to resemble a D. Doug, please don't be afraid to try to wipe it off. If it's real, it won't wipe off.
DON'T try to wipe it off. DON'T play with the coin in any way. You may have a retained letter debri from the D mint mark. The D is incused in the die and makes a raised D on the planchet, coin. The die has an incuse mint mark which, when it strikes a coin causes the planchet metal to fill the D under pressure. Sometimes the mint mark, in this case a D, becomes filled with grease and metal. This results in weaker and weaker D on the cent until it is no longer visible on the coins struck. Study the 1922-D and the weak D variety which eventually became the no D weak reverse varieties. As the metal chips, flakes, etc. and grease press together under the striking pressure, the debri becomes very hard. Sometimes the filled material in the mint mark dislodges and falls out on a blank planchet anywhere and when the die strikes the hard debri on top of a planchet, the debri makes a mark in the die. It could be just a mark, it could be a complete letter, it could crush to nothing and be a struck through grease or material. Remember under striking pressure, heat is created and the dies get hot. Your HOT D may have fallen on your struck coin and your coin was ejected before a blank came into the collar die. Now that the filling debri had fallen out of the mint mark area, the next cents have a D mint mark again. Error collectors such as Paddy or Fred Weinberg can explain this better. What a retained dropped letter debri error is called, others will know. But DON't tamper with the coin. Research first and ask the experts.
You are not seeing this correctly, The cent in question is a Philadelphia minted Lincoln not a Denver. The only possible way this is a drooped letter is from the D in united. And to be honest this could easily be clarified by sending to an expert for about 10 buck, the photos are less than convincing either way.
Thank you for the correction. Yes the only way it could be a D is from the D in United doing the same thing. Filling with debri under pressure and falling out. Better pictures or in hand review by an error expert is the best solution. Thank you.
Doug you wrote "I sent the penny to them" You did not write that you had sent a photo to them. Was that a error on your part?
The D in GOD is a different size and style than what you are seeing.. A thin piece of Copper plating would not make an incused form into a Steel die.