These are found inside the uncirculated mint packages to indicate which mint they originated from. (as if the 'D' wasn't enough to indicate which mint it came from - or the lack of one ['P']). Since Denver and Philadelphia are the only mints that produce the uncirculated sets, these are the only ones you will find.
Collecting since 1972. Learning from then on
"Tell me and I forget. Teach me and I remember. Involve me and I learn." - Benjamin Franklin
My Dansco 7070 type set coin count: 37/76, My American Silver Eagle Proof Collection - 1986 to Present
My Dansco 8176 Eisenhower-Dollar Album, My Dansco 8166,8167 Kennedy-Half Dollar Albums
My Dansco 8140,8143-8147 Washington-Quarter Albums, My Dansco 8125 Roosevelt-Dime Album
My Dansco 8113,8114 Jefferson-Nickel Albums, My Dansco 8100,8104 Lincoln-Cent Albums
"Tell me and I forget. Teach me and I remember. Involve me and I learn." - Benjamin Franklin
My Dansco 7070 type set coin count: 37/76, My American Silver Eagle Proof Collection - 1986 to Present
My Dansco 8176 Eisenhower-Dollar Album, My Dansco 8166,8167 Kennedy-Half Dollar Albums
My Dansco 8140,8143-8147 Washington-Quarter Albums, My Dansco 8125 Roosevelt-Dime Album
My Dansco 8113,8114 Jefferson-Nickel Albums, My Dansco 8100,8104 Lincoln-Cent Albums