I think it`s easier and thus cheaper to make this way.. You have easier acces to some parts with your mill or lathe.. If you really want your reed to vibrate more freely, you'd best make that gap even wider.
I had a good look at a couple of DVs recently. I don't think this feature makes manufacturing any easier, the bore is drilled straight through to the baffle on these, there's no need to get any tooling in there.
As to what it does, a longer window does seem to lessen resistance. Acoustically, it will make the chamber a little larger, which can be a problem for high baffle pieces like this.
I doubt that it has a beneficial effect on reeds w/r/t swelling. If it had, everybody would have copied it 75 years ago instead of Meyer copying everybody else and removing the feature from their next model.
One thing -- regardless of how good or how cheap this is, it's an obvious knock-off of a patented design. We shouldn't buy them and I won't work on them.
