That knurling that you see is an artifact of the lathing process itself.The 18" is early 1970's and the hats are 1960's.
Even though the stamp is similar to a '50s stamp, I though '70s as well beacuse of the lathing. I've always thought that type of deep even lathing starting on the bell was indicative of a later period than the '50's, and since that stamp was also used in the '70s I jumped to that conclusion. Could you clarify what the determining factors were for you?