This is purely a guess but just because they stopped manufacture doesn't mean that they don't have a shelf full of barrels left over from when they did. Again, just a guess on my part. And if one were truly interested in this conversion, it would certainly be worth a call to find out.
From what I'v seen it can be hard to get them to work on discontinued items so they might not do it even if they did have barrels.
That said, its likely they'd put a 38/55 barrel on it that you could get rechambered for the 375 or if you handload, again from what I gather, I think you could load up pretty close to 375 velocities.
TedH , It's a very easy job if you can find a barrel and have the right tools. I found a 375 and a 356 barrel on ebay last year. I'm very happy with the 375, still looking for nice 336 for the 356 barrel. The 375 started as a 30AW from walmart. HYDE
Hello djh , you almost had another shot at that barrel this winter. My union was ( still is ) on strike for the last 6 mo. I found a new job at a better shop about a month ago. It was a long winter with NO INTERNET untill two days ago. We got through without having to sell any of my guns but , it was close. HYDE