Thanks guys. That confirms some of my suspicions. I eventually found a good description of 'Stehkessel' in EJ's technical series 'Die Dampflokomotive - Technik und Funktion, Volume 1'. Loosely translating it says that "part of the rear boiler section includes the Stehkessel on the outside and the firebox on the inside. The name Stehkessel came from the very earliest locomotives where the rear boiler was clearly distinct from the main boiler, stuck out above it and was oriented vertically."

From the diagrams the Stehkessel clearly forms an outer 'skin' around the firebox with an air gap between the two partly occupied by numerous bolts and pipes. So on the early engines it was indeed a 'vertical boiler', but later became the outer part of the firebox.

I've found no more on 'Umlaufblech' but the suggestion that it is the steel running board around the frame fits with the context.

Many thanks,

Paul, of (formerly Prussian-occupied) Lower Saxony