I cut it off an old mexi-conn. Yes, it's soft soldered. There was a nice thick piece of cork behind the Bb key, so I removed it and just soldered the extension on. I could play it the way it was, but I am so much faster with the 3 in a row setup.
I figured it is easily reversible, and it's not an immaculate sought after sax, so it was worth a shot!