Almost forgot to answer your question. The bushing is the ring or threaded protector that goes on threaded connector after the lock ring to protect the wire from abrasion. Often plastic but can be found in metal with a plastic liner and ground connector. But the simple plastic ones will suffice. Look at the picture of the panel I posted and note where the feeders enter the panel. The gray bushing is what you need on yours. Required for 4 AWG and above
As for your aluminum conductor gauge. Older stuff was slightly larger in diameter because the where simple round strands bundled together. Modern stuff us compressed together to reduce the space between the strands. This allows packing the same amount of aluminum strands in a smaller space. Take a look at the shape of each strand and you'll see it's no longer round. So the one that is slightly smaller in diameter to your existing feeder is likely the same gauge
