As I understand it, Apollo 15 was originally meant to be another H-class mission. When the program was shortened it was cancelled as originally planned and made the first J-class extended mission. This probably moved up all the J-class hardware by one mission.
The J-class LMs had to be built as such, so LM-9, which was to be the H-class LM for Apollo 15, was put on display at Kennedy Space Center. As far as I know this is the only complete, unflown LM originally intended for a manned flight; LM-2 on display at the Smithsonian was originally built as a backup for LM-1 which flew unmanned on Apollo 5.