The reverse dies for Lincoln Memorial Cents was fairly consistent year over year. This reverse had a evenly spaced AME in "America". Then, in 1992 they prepared new dies for 1993 with a slightly altered design. The AM were now touching at their bases. The dies were intended to be introduced in 1993, but apparently someone used it on some 1992 cents (in both the Philly and Denver mints.) Next, in 1994 the mint began to use the old style wide AM on the proof cents, while keeping the newer close AM for the regular business strikes. In 1998 and 1999 mix-ups occurred in both directions. Some proof coins were minted with a die intended for business strikes and some business strikes got the proof reverse. In 2000 the proof reverse was again used on the business strikes, but no known proofs have been found with the business strike reverse.
There are 7 varieties that can be found: 1992 P with a close AM 1992 D with a close AM 1998 S with a close AM 1999 S with a close AM 1998 P with a wide AM 1999 P with a wide AM 2000 P with a wide AM