I don’t understand the purpose of your post, if not for purposes of a test question to see who gets it right.
In your initial post, you ask if the mattress on a 350/390 is custom or standard, and your next post says, while replying to someone else, “It’s not a standard bed shape. It’s wide up top and tapers towards the end of the bed. An island bed. If you have a 350/390 you’d know the shape.”
OK...so you know it’s custom, not standard, from the get-go. That brings me back to your original post...what are you trying to learn here?