Hi,
I'm uncertain about what constitutes a legitimate rear light cluster. Someone suggested there is a radius issue here. Is that correct? I have some Lucas light clusters which are mareked E11 but with very angular edges.

I also have a pair of rounded clusters with no Emarkings at all. Which (if any) would be best to use?
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top ones I'd say, the E marking is a must and the edges look OK on them.

Not as long as it complies to the standard, You'll need a reflector with those as there doesn't seem to be one in there.