>There is an extra slot at the top of each column of
>18 tapes that seems to be the perfect spot for a cleaning tape.

The 3583 Operator manual on page 6 defines that slot:

    The nonaddressable slot located at the top of storage columns 2, 4, and 5
    is reserved for future use. The bar code reader cannot read the label on
    the cartridge if it is stored in the nonaddressable slot.

Page 53 says:

    The cleaning cartridge must be stored in one of the available storage
    slots within the library.

It identifies the cleaning cartridge by virtue of its "CLNI" volser prefix
(page 114).

 Richard Sims, BU