The side boards have some hardware to install on them before assembling the frame of the machine. Most important is the motor mount plate and shaft end mounts. You may also wish to install the microCarve A4 name boards at this time, though I left them off until later, so that the side boards would be lighter and easier to handle during assembly.
Motor Mount Plate
As with the motor mount for the table axis, the plate for the bridge axis has a side that goes out and a side that goes in. Looking at the screw holes will tell you which is which, as with the other plate. Look for relief cuts, like countersinks, that make it easier to insert the motor mount screws. There are two plates, mirror images of each other, one for each side. Matching them up to the side boards will make it obvious which is which once you've identified the "out" side for each plate.
Opposite the motor mount plate on each side board is a small plate that the shaft mounts rest on. There are two of these, identical for all purposes so far as I can tell.
Just as with the bearing blocks on the front board and back board, these metal bearing blocks need to be installed with the larger part of the center hole facing out from the board (toward the center of the machine, once it's assembled.) Above and below this hole is where the shaft mounts get installed.
The shaft mounts get screws put in, as before, then those screws are passed through the small plate, the side board, out through the motor mount plate above and below the leadscrew hole, then washers and nuts are put on. Once again, don't make these tight, you'll want to adjust things later.