Shane,
The reprint of the 1930's Zeiss catalog doesn't list a 840mm Apo-Tessar. Here is the following description from the catalog on the Apo-Tessars:
The Apo-Tessars are well corrected with respect to all those qualities which affect their performance as process lenses, and their residual errors are reduced to within very narrow limits. Focal lengths listed: 24cm, 30cm, 45cm, 60cm, 75cm, 90cm, 120cm, (all f/9) and a 180cm f/15.
I have the 30cm (300mm) and the 45cm (450mm) versions of the Apo-Tessar both in brass barrels. The 450mm is one heavy lens so I'd make sure the one you are considering comes in a aluminum barrel.