The center portion of the backplate is designed for the standard 11" separation used for the bands holding doubles cylinders together, even small AL40s such as these.
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Attachment bolts on the bands are used to hold a buoyancy compensating wing.
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The plate has sets of holes 11" apart, or holes and a slot. Threaded into the plate is the webbing harness, to which needed gear is attached .
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When diving a single tank, the plate's 11" holes can be used to attach a singles wing and a single tank adaptor instead of the doubles. Or slots in the plate allow cam bands to pass through a singles wing and attach a tank directly.