No, that's not normal. Someone has stripped the threads in the cover plate. I'd suggest removing the plate and having a welding shop braze half a pipe coupling to it so you can screw a pipe plug in securely. That would also be a good excuse to clean up the differential housing with some solvent and refill it with fresh gear lube after the cover plate is replaced.
If you choose to use bulk lining material, simply measure the length of the original linings and duplicate them. Clamp the new lining to the brake shoes, drill the rivet holes in the proper place by drilling from the brake shoe side, and countersink the holes in the lining deep enough to prevent the rivets from scoring the drums. A depth stop on a drill press quill will prevent drilling those counterbores too deep. You might be able to find direct replacement linings, but making them from scratch isn't difficult. Some brake linings are different length front and back, so be sure to take pictures and make measurements of everything before you disassemble the brakes.
Jerry
