Just back from a 4 day 1200 km car trip with the Integrale, but managed to read the post and found this on a Morgan...
Looks like a good solution with the inne cap beeing lockable.
The inner cap sealed very good too, after the Morgan had been standing in the sun, there was a huge overpreasure in the tank that came out when we took it of.

Closed
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Open with lockable inner cap
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Open without inner cap
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Inside of lid
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Griff, have not got the final invoice yet, and have had enough with the Norwegian B-roads, the fjords, the 5 meter snow on the side of the roads and the splendid weather.... Will bring the Mac over so you can take a sneak peek on what you are missing out not coming over this year.... But it will be there for years to come, so no worries.

Regards

René