Seems from what I have been searching. Early 60s BOP cars got the 5x5 pattern. At least that's my understanding. Internet searches haven't really net much info accept for a post or two on the HAMB that actually were quite helpful that popped up in the search results. Im not picky on the size. From what I understand, 15's are more common.
__________________
Chris - Ramsey, MN.
Dear Optimist, Pessimist, and Realist.
While you guys were busy arguing about the glass of water. I drank it!