[Gunner]

ERAs have the inside cap. I didn't understand the reason for these until I installed an original GT-style flip-cap on my Mustang. It came with parts to use it as-is, or with an inner cap. I use the inner cap because almost any hard jolt can open the flip cap, which, facing to the rear, would spew fuel on acceleration. Or a moderate rear-end bash.

The LeMans caps are better designed but it's still the same principle. For safety (and also to meet many state requirements), I'll take a nice, tight, secure twist-on under the beauty cap.