Just got some Barber quarters and going through them. These are my first and I'm not real familiar with errors on them. This one looks like a D mm behind the O. For this 1903 it would not be likely, since the Denver mint didn't start operation until 1906. Die clash? Look like maybe even some other clash marks, but with it so old it's hard to tell. Probably just be PMD. Thanks for any help.
I see what you are talking about but I think the elevated part of the coin surface (and color differences) is just making it appear like a "D". As you stated, the Denver mint wasn't producing coins yet and didn't start to unitl 1906 (3 years later). If anything it might be another O mint mark that got squared on the left side.
There are no major varieties involving mm for the 1903 O barber quarter. That being said, the O mint quarters are often found very weakly struck in the tail feathers, feet, and mint mark. When you add a lot of wear to that initial weak strike you can occasionally see some spread in the details, which is what I think is happening here. That depression is just a byproduct of circulation, not enough to be considered PMD, but odd enough that it makes you wonder how it came to be.
Look at the surfaces of the coin around the "O". That is what gives the illusion of there being something there, but there is no mintmark, letter or anything else. As mentioned above, it's an optical illusion due to the rough surfaces.