Is it available commercially?

Thanks for the pics. I assume it's a custom-made clamp from "Vincent van Eerd at Eerder Metaal & de Brompton Winkel". I'll drop them a mail and ask 1) if they could make more and 2) how much it'd cost.

I'm also thinking of sawing off the lower part of the fork, as someone did here on a Campi derailleur: https://www.facebook.com/groups/Lond...2%3A%22R%22%7D

For that reason, I do recommend getting a derailleur whose fork is screwed together instead of riveted.