[SS427]

Jim,
It is based on both plant and build date. The difference is shown on box 91 of the build sheet. You should see 770, 775, or 323. Some are followed by an FJ code. 775 is deep center with bolt on backs, 770 or 323 is for deep center with clip on. Generally, most plants were changing from clip ons to bolt ons about mid March of 1970. Because of loss, many people are using the bolt on caps for their cars regardless of build date.
Rick