Made for cold locations to start easier. I have some of these old Wotan HQL's buried away in the back of the container...great to see the plant mark on this, "y" (which I cannot find a plant listed for!) The
"KX" would be the date code. I will see if I can decode it...
It is a wierd old code...
The first letter in always lower case and denotes the plant and the quarter of the year...so four different letters represent the same plant (in this case the Nonnendamalee 44 plant in Spandau where "v" means made at Spandau in the first three months of the year, "w" means made at Spandau in the second three months of the year and so on....) The second letter "K" gives the year from a look-up table, which is semi randomly assigned to the year and its case indicates the half of the year...lower for first half and upper for second half. So this was made in the second half of the year. The last letter indicates which of the three months in the quarter it was made, X = First, Y = Second etc. So it was made in the first month of the last quarter...so October. The year letters cycle in a semi-random way in a look-up tablr with k = 1956, but this lamp ain't that old...I would very likely have yttrium orthovanadate phosphor (look at its spectrum). The last year in the look-up table is 1973, so assuming the table repeats with the same pattern (which looking at later tables, it does) "k" returns in 1977.
So.....Made at 44 Nonnendamalee, Spandau, Berlin, October 1977!
"HPL", (I assume meaning "Hydragyrum Phosphored Lamp") was the Philips equivalent for the MBF/U, but it was licenced to Philips, so Osram had to think up something else...well move the second letter along the alphabet one space and get "HQL". I think Radium used "HRL" and Sylvania "HSL" by doing the same and getting a middle letter which matched the initial of their brandnames.
Some of my old Wotan HQL's are made in Italy...I suspect at the Bari-Modungo plant which now does CFL.
Co-Ordinates Spandau... 13.251389 52.5365
Bari-Modungo........... 41.104851 16.774289 Plug into http://itouchmap.com/latlong.html to see plant.