Lee Filters LEE 100 System Instructions page 8

To fit a new filter block, make sure the word LEE faces up and the open
side of the block faces outward.
Clip a new block over the small turrets on the backplate and push firmly
until you feel a positive click.
Place the appropriate size of grey cover plate into the open side, making
sure it is in the correct orientation, to complete the assembly. Keep all the
spare parts somewhere safe, as you will need them should you wish to
reconfigure the holder at a later date.
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