My guess is that someone at the PRW factory stamped a set of 1.50 rockers with the 1.60 ID and then they got boxed and sold as 1.60 rockers. PRW gave me a set of their steel rocker arms when they first came out for a magazine article, but the rockers were a timebomb so I never used them. The rockers had ribs around the roller tip that were too big. The ribs hit the valve lock before the roller hit the valve stem so the load was all being carried by the valve lock. I never even started the engine when I saw that. Just boxed up the rocker arms and sent them back and never included them in the article. I assume they fixed this design flaw but I haven't looked at a set since.