<Fascinating the things you discover thru mere conversations like this. I never noticed how similar the green mounting thingy & the black thingy where the fuel filler flap's bowden cable moves into to lock the fuel filler door, look alike. From the pics, it almost looks like the only difference is color...go figure. 
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Grateful for the tether on my e39 & e53...but also grateful that BMW designed a place to rest the cap on the fuel filler door, as well...especially if/when the tether breaks off.
My e38 didn't come with a "tether"...back then, there was only the fuel cap resting place designed on the fuel filler door:
{E38...no tether and slot for resting fuel filler cap}
{e39 had tether and little green mounting hole}
{e53 also has tether and little green mounting hole}
