The KT81 is the first M-OV kinkless tetrode audio power valve with a pressed glass foot. We can not find details of how it performed in the market place, but it was not a valve produced for many years.
The Marconi paper label. No doubt the Osram valves came off the same production line and differed by only the paper label.
The coating inside the glass obscures almost all of the inner detail. The control grid is wound on copper supports and has heat radiating fins at the top.