Based on feedback from the comments and some emails I'm having second thoughts about revising the Miskatonic Antarctic patch.
As I said before, I'm pretty sure Lovecraft meant the aircraft used by the Dyer expedition to be the Dornier Do-J "Wal". On the other hand, the specific mention of securing the landing skis almost certainly identifies the plane as the Dornier Do-B "Merkur II".
From a realism standpoint I think the scale tips in the Merkur's favor. The story specifies that the plane has a range of at least 800 miles and a ceiling of 24,000 feet. The Do-B can hit those targets with relatively minor modifications, like swapping out the BMW VI engine for a BMW VII with a supercharger.
Ironically, the BMW VII is the same engine Wolfgang von Gronau used on the modified Dornier Wal he flew around the world shortly after Lovecraft wrote the story. With a pair of them the plane easily had a range of 1000 miles, but it's service ceiling was only 11,500 feet. The problem with climbing higher was the Wal's tremendous weight. Even with superchargers the engines didn't have the performance to hit the 24,000 ceiling "At the Mountains of Madness" demands.
Frankly, I don't think Lovecraft cared a whit about the flight ceiling issue, but getting the Wal to meet the criteria he sets out would take some major work. The story contains several mentions that the plane was lightened for Dyer and Danforth's final flight, but I can't see the aircraft's performance more than doubling just by stripping the interior. Beyond that we could speculate the expedition aircraft were already considerably lighter than the stock Wal. The easiest approach would be to cut the plane's weight by replacing some of the steel frame with aluminum. That's still a pretty radical move, but given Pabodie's canon expertise in lightweight aluminum frame construction it's not beyond the bounds of belief.
Ultimately, what plane was used seems to come down to taste. Rely on logic and historical accuracy and it's the Merkur II. Trust in Lovecraft's original intent and it's probably the Wal. I can see equally valid reasons for embracing either.
So what do you think? I've already invested a considerable amount of time developing prop photographs and imagery using the Wal. I'm willing to put the same effort into redoing all of it with the Merkur II.
Update: I want to thank everyone who has weighed in, both in the comments and via email. Even the ones pointing out that I'm being a bit obsessive. Heh. I'm in no rush to make a final decision and ultimately it only really matters for the next run of swag. That won't be happening until after the first of the year, so there's plenty of time to fiddle with things.