IBM 3590 - Manual (Page 32)

Note: The 3590 tape drives that are capable to receive the Extended High
Performance Cartridge Tape are labeled 2x on the back of drive chassis.
For B Model drives, the 2x label is placed below the serial number
label. For Ultra and E Model drives, the 2x label is located below the
respective Ultra and E labels which, in turn, are located below the serial
number label. See Figure 4 for an illustration of the tape-icon label.
The 2x information is available in Table 3 on page 10.
Figure 4. 2x Label Icon
12 3590 Operator Guide