Complex sentence: Earlier, the post office did not use any specific numbering system but this changed when they started using a rather simple system on their leaflets: xxxx--y where xxxx is the year and y is the issue number within that year.
Split into simpler sentences: Earlier, the post office did not use any specific numbering system. This however, changed when they started using a rather simple system on their leaflets: xxxx--y where xxxx is the year and y is the issue number within that year.