I have a theory conceived from my research on this part from browsing several Japanese collecting forums. I believe with usage, the block begins to look more like the original part in the photo due to the mechanisms inside the gun that cause the block to slide back and forth. Often times the forces are so strong on the block that they often crack or break the block in half. There had been a run of brass reproductions produced by a member of one of these Japanese model gun forums to replace the damaged aluminum blocks.
With that said, the majority of unused blocks that I found appeared much like those on the ROTJ E-11 prop, which is reflected in my reproduction.