Annular buffers in the diagram are tagged 1,4 and 2,5 and 3,6. Why are there 2 part numbers for each of the three items?

There are some serial number breaks as there are different mounts for different years. https://www.lsengineers.co.uk/catalog/product/view/id/11659 - item 4, https://www.lsengineers.co.uk/catalog/product/view/id/11660 - item 5, https://www.lsengineers.co.uk/catalog/product/view/id/11719 - item 6