Hi. I recently bought an old lantern (1920s). It has a bulb in it that I am keen to determine the age of. I've trawled the 'net and found some old papers which give Osram date codes but my problem is that I don't know which number on the bulb is the date number so I'm unable to date it. I also found your gallery but was unable to find the information I need.
The bulb is a ?' squirrel cage' filament bulb and it measures approx. 14cm in length (including the bayonet fitting) by approx. 6.5cm at the widest point of the bulb. It has some writing on the end of the bulb glass which isn't perfectly clear but I think reads as follows. There is a central OSRAM name, above is written 'PATENT GEC LTD' and between the two is difficult to read but it could be a '50'... The '5' of this is a bit of a squiggle!
Below the OSRAM is 240v 60w and just below this is a number 17.The very bottom part reads 'MADE IN ENGLAND'. I appreciate that bulbs break and am therefore not expecting the bulb to be as old as the lantern but it would be interesting to find out potentially in which era the lantern was possibly last
used.
I've attached a couple of photos to aid in identification but it's really difficult to photograph the writing on the bulb. Can anyone help?
Thanks,
Steve.
 

The date code is 50, which probably means February 1938 or even as early as 1930 (the code runs on an 8-year cycle so it is difficult to be more precise).  The 17 just indicates the production line number within the factory.

Thanks James. So the earliest it could date from is the 30's. Can I assume that the 8 year date code cycle continued after this? If so, do you know what is the approximate  most recent date that this bulb may have been manufactured? Also, does the number 17 give any clue as to where it was produced?
Thanks,
Steve.

Indeed that code continued in use until about 10 years ago, but I think 1938 is probably the latest date for your lamp.  By 1946, as far as I know, they would not be still printing the Patent notice on the lamp.  It was made at Hammersmith in London, see link here.

Thanks James.... Very helpful.... Especially the information regarding when Osram dropped the GEC 'Patent' etching.