Thanks to all for the ideas and input (posting and PMs). I have some other suggestions to follow.
We're thinking the engine is about 6.3 liters if that helps anyone.
I have a small video/audio clip, but haven't figured out how to embed/post yet. Here's another photo in the interim.
If anyone has any historical documents about the 1921 Avus race itself, that would be excellent as well. Anything - news accounts, results, entry lists, photos, etc.