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 shimif you want to fit the uIDE card internally, although space is very tight under the keyboard (the processor sits directly behind the expansion port) and . You may also fit the RHS shim to a 6128, on the following provisos: * a) the strain relief bar of the Z80 bus cable that plugs into the shim must be removed (otherwise it's fouls the rear case).* b) the Z80 bus cable might not known yet whether be able to bend round to allow internal fitting and* c) the Z80 bus cable, when passed out of the expansion port slot, may get in the way of any expansion you have fitted (and it will fit properlymay not, depends on the shape of the expansion unit. PCW 8000 machines also require the LHS shim, so because the RHS shim fouls the inside of the case . The good news here is that you can be fittedpass the Z80 bus cable through the case at the right hand rear corner (where the keyboard cable plug is) without modifications to the case. If you look closely at the join, you can see there's a short section of case, right on the corner, that has a flat face. Route the cable through this section, and don't tighten the screw on that corner up too hard.
'''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.