The leather on the chinstrap in question certainly looks new and so does the stitching. I've never seen such mangled, ineptly installed rivets on any WW1 item. I wonder if original metal M18 chinstrap components were used to make the new strap.
The chinstrap posted by Steve is an excellent example of what an unquestionably good one looks like.
The M18 helmet looks to have been cleaned, the liner removed at least once, and someone had applied transfer decals to it at some point for reasons unknown. At least the liner looks original to the helmet. It's my impression that an attempt was made to restore the helmet to a more collectable condition by removing the decals and installing a new chinstrap.