Nice car, but it's not a Landau Sedan (Model 328), it's a Model 320 5-Passenger Sedan. The clues to it not being a Landau Sedan are the narrow "D" pillars and lack of landau irons. (It's also technically not a Classic Car, i.e., one recognized by the CCCA, a common mistake with the overuse of "classic car" in today's lexicon).
Sorry I don't have a better image, but here's an ad for the '28 Standard Six Landau Sedan (Model 328), with a greenhouse quite different aft of the rear doors from your car.

Too bad it's not the Model 328, as apparently few of the Standard Six Landaus have survived.
Here's one with a pretty heavy-handed, overly padded landau roof treatment posted on a forum in 2011...

Hope your car does well at the auction.
TG