I bought mine only a couple months ago, and it had the thread pattern that works with the existing extension from S&J - maybe I got lucky and it was old stock...
As far as the legality of it...
I can only fit 5 shells of 3 inch magnum in it now (with the extension) - close to, but not quite 6 - and that's what keeps it on the legal side, because the gun is chambered for 3" magnums, and that's what's used to determine maximum legal mag tube length. I can only fit 6 shells of Federal 2&3/4" 00Buck or 2&3/4" Score HV slugs in it, but I can barely squeeze in 7 of the 2&3/4" Score target shells (they're actually quite short for 2&3/4" shells). There's actually a lot of variance in length of shells even within a given chamber spec and shell manufacturers.
The Benelli crowd are doing the same thing, only at 3x the cost, to squeeze 7 shells into their tubes. Some guys custom cut their shells even shorter and go all the way to 8 or 9 + 1, and aren't breaking the law.
It's the same legal jiggery-pokery that means we can use the .50 Beowulf mags in our 5.56 AR's, and still be legal.
The Hatsan rep is mistaken on the legal side. Or is being misleading. Hatsan outsources some of their production to other factories in Turkey, and there might be two different specs coming out at the same time.