This 2.80" O.D. Stainless Vented Gas Cap was designed for 1966-1970.5 models only.
Early model Broncos prior to the introduction of the evaporative emissions system with charcoal canister have a different sized fill neck than later versions.
CHOOSE THE CORRECT CAP
In picture #2 above, the cap on the left fits the 66-Early 70. That picture shows the neck on this cap.
The cap on the right is the Late-70 to 76 (part #9747)
where the evaporative emissions system was used. The difference is the later 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.