Complex sentence: Appointment to the Assay Commission was eagerly sought after--for one thing, commissioners received a commemorative medal, different each year and, with the exception of the 1977 issue which was sold to the general public, extremely rare.
Split into simpler sentences: Appointment to the Assay Commission was eagerly sought after, in part because commissioners received a different commemorative medal each year. These medals were, with the exception of the 1977 issue which was sold to the general public, extremely rare.