Manufactured by TA in the '40s and '50s, and popular on you know who's bikes. As the derailleur slides outboard, the distance between the derailleur and the BB gets slightly longer (trigonometry), so the cable gets tighter and can bind. This shifter solves that problem. For €590.
Quite a find, but the price was staggering! Seller has since taken down the listing, but you can still view it:
https://www.ebay.fr/itm/SHIFTER-LEVE...T/203009964787
Guessing they did a deal off Ebay. But for 600 euros, I would file one from a hunk of aluminum myself! (and now I'm going to, since the seller was nice enough to post some clear pictures with a measurement or two).