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SUBMIT YOUR IDEAHere's the solution - kachow!
Didn't really enjoy
My Solution (no composite keys required) matching accessories to frames. simple hint: go vertical.
Added section creating the requested output.
I followed a similar approach as @RolandSchubert - I took the last formula tool out (thus, concatenated using comma only), as I did not think the field with [] was really needed for the solution. It is extra to have, but it does not drive the solution. Thank you, Roland, for trailblazing the solution on this!