Hi Rigo,
You have here a very fine lamp! The OSIRA name was a trademark employed by the General Electric Company of England on all of their gas and vapor discharge lamps made between the 1930's and sometime after the end of World War II. I believe their first Osira lamp was the hard glass mercury source made in Wembley in 1932, the world's first fully sealed atmospheric-pressure mercury lamp for the matter. After the defeat of Germany in WWII the "Osira" name changed to "Osram".
As for your lamp, this item is known as a Compact Source Lamp, simply a high pressure short-arc mercury lamp. The first version of your lamp appeared on the market just before the outbreak of the war, and was develloped for projection purpose since it was found that mercury arcs could be two to four times brighter than the best carbon arcs - without saying that the maintenance was also much easier.
Your particular lamp belongs to the second generation of the CS family that was employed in the Royal Air Force's "Bomber Teachers" - i.e. projectors displaying a moving panoramic aerial view of places to be bombed. This system was used to instruct bomber pilots about their targets.
Because of the veil of secrecy that shaded techniques develloped during WWII, this particular lamp was featured in a progress report of the GEC journal only in february of 1946, so it is hard to say exactly when this lamp was develloped and put to use.
About the specs, this source seems to be a 500W lamp. If this is the case, then it operates at about 10 atmosphere of pressure with a power loading of ~90W/mm with a 5.5mm arc length. The voltage drop is about 12V and has a current rating of 40-45 amps.
At last, judging from the date code on the stamp, the lamp may have been made in september 1945 or 1953.
Best regards and keep up the good work with your collection.
Max
