Filled Letters , Numbers?

"Clogged" letters are a simple matter of small parts of the die chipping away with the reslt that that part of the coin doesn't get a full impression. This is very common for the dates you mention.

The reason why the position of the mintmark moves from coin to coin is because the mintmarks were placed into each working die by hand. As long as the mintmark was somewhere below the date, inside the rim, and in front of the vest it was considered well placed and went to the minting press. All of the mintmarks that fit in this category are considered normal and are not worth a premium.