Don't know if this is just from the coloring or because of age, but I really don't see anything of Lincoln sitting in the memorial.
Neither do I. Maybe he stepped out for a coffee break. Seriously, though, you'll need to post better pictures. It may be due to an over polished die.
41 years have passed since that Cent was milled. From the beginning it could of been a grease filled in the area where Lincoln was sitting and it created a weak image. From that time to today it could of just been worn down. There are thousands of Cents that look like yours. Not a biggie Your 1974 Cent does look a bit MAD! Misaligned-Die.. See how the rim is thicker at the top than at the bottom?
I don't see Abe either, but then that'd be like looking for a field mouse in a satellite image. Seriously though, that part of the die is very shallow and gets polished away very easily. Or it could be as Paddy said, grease filled. If you look at enough Memorials you see it frequently.
The Lincoln figure is in a protected area of the coin. For it to disappear from "wear" would require that the columns on either side to be worn as well. That level of wear simply is not visible on the columns for that coin. As such, I see this simply as the result of a grease filled die which, as you've pointed out, can be quite common with the Memorial Cents.
