"Solid" mintmarks are common in most denominations. There are two causes. A minor die break causes the raised part of the mintmark on the die to fall off due to heavy use during the striking process. This die fatigue can obliterate the post creating a solid mintmark. A die chip may also remove the central "post" in the die face, leaving a solid loop in the letter on the coin's surface. It's not a die variety, and isn't worth much usually because it isn't uncommon but might occasionally sell for low amounts. There would be hundreds or thousands of them out there. If the post broke off half way through a run, you would have as many filled in mint marks as you would have regular mint mark. Off the top of my head I believe it was the late 1970s that I remember seeing a ton of them.