Chrome Vented Gas Cap for 1971-1976 models.
Earlier year models have a different sized fill neck and this one will not fit those models. This cap is the cap on the right in the photo below.
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The cap on the left is the 66-70
(part #9748) on the right is the 71-76. The difference is the 71-76 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 you old cap to determine which version you need.