Jrichker,
I think you are mistaking shaft mount rockers and rail type rockers as being the same thing, they are not.
Rail type rocker arms, as have already been said, have little ears that help keep the rocker tip center on the tip of the valve stem. If you look at the following picture, you can see the little ears sticking down past the roller tip.
http://static.summitracing.com/global/images/prod/large/cca-1450-16_w.jpg
I'm sure you know what shaft mounted rockers look like, so I won't waste your time by posting a picture of them. But, Ford started using the rail type rocker arm in 1966 and used them for quite some time until the introduction of bolt down pedestal type rockers much later.