This is what is stock on a non roof rail 1958. I used simple sealing washers and silicon when I installed my towers. You will note the white plastic tubes that stand proud of the roof gutter. I had to slightly enlarge the sealing washer hole to fit over them, hence a little silicone as well. It's a pretty silly system as the plastic piece in the first picture simply caps the rubber gutter rail inserts. I left the rubber insert out between the two towers. I may glue is back, I but I would need to trim it a little bit first. The third picture shows the attached tower base - you can see the stainless sealing washers underneath the base. My inclination is to simplify a plate over the rubber ends that one simply bolts through rather than the bolted on wire clip and the plastic snap on cover that attaches to it.
