Barcus Berry 1320 - Manual (Page 7)

To mount the output jack on your instrument
tailpiece, remove the screws from the clamp
assembly and place the U-shaped member
beneath the tailpiece. Position the other part of
the assembly (which holds the back) on top of the
tailpiece directly above the U-shaped member.
Align the holes in the two parts, replace the
screws and tighten until the assembly is firmly
secure in place.