These small cards are inserted between the Z80 and its socket, and present all 40 pins of the CPU via a 40 way header. This header is the "Z80 Bus" that the uIDE cards connect to.
Choose the one that fits best inside your machine. A CPC6128 needs the left hand shim, although space is very tight under the keyboard (the processor sits directly behind the expansion port) and it's not known yet whether it will fit properly. PCW machines have more spacealso require the LHS shim, so either shim should fitthe case can be fitted.
'''Important:''' Check your machine has a socketed Z80 before ordering a shim (you should open it up anyway to decide which shim gives the best mounting solution). If the Z80 does not have a socket and you are not willing to desolder the Z80 and fit a socket to the machine, then a shim is not the solution for you.