Fieldpost mail for the Austro-Hungarian Navy vessels on the Danube in WWI

The Danube was an important transport highway, as well as a border during WWI, and was guarded by a detachment of the Austro-Hungarian Navy base din Budapest. This picture postcard was sent from a crew member of the small patrol boat S.M.S. Viza to Budapest. The card carries the large double arms cachet in red of the boat (representing the dual Monarchy of Austria and Hungary), which acted as a censor mark, and a K.u.k. Feldpostamt 299 ‘b’ field post datestamp known to have been in use in occupied Romania in 1917.