This 2.80" O.D. Vented Gas Cap was designed for Late 1970-1976 models.
Earlier year models have a different sized fill neck and this one will not fit those models. This cap is shown on the right in the 2nd photo above. The cap on the left is the 66-Early 70 (part #9748)
on the right is the Late 70-76. The difference is the Late 70-76 cap has a longer reach into the filler neck.
Make sure you order the correct one for your Bronco. Because many Broncos have been taken apart and put together with parts from various years it's better for you to look at your old cap to determine which version you need.