I noticed this as well. I reached out to Ed Byrnes about it, who believes the elevation cap is original to the rifle. He mentioned he’s seen grooved caps in this range. The cap has also noticeable speckled wear that is uniform with all the steel parts of rest of the rifle. It seems like an arbitrary piece to leave uncorrected.
Regardless, it’s not something I would change on the rifle. The gentleman I spoke to said his father returned home with it wrapped in a wool green army blanket. I know the case of the “buy the rifle, not the story” but he was sincere enough in the sense that he had very little understanding of M1rifles. I took a chance buying the rifle because his lack of knowledge prevented me from seeing many parts in photographs. He didn’t want to take the rifle apart so I took a chance with the internals and the lower band pin.