Fascinating stuff about Siemens!
I must say, before I knew it was Siemens I'd always interpreted the poster to be an advertisement for light bulbs, mainly because it shows a picture of one. But it certainly emphasises China's transformation into a modern, industrialised nation, a change which is littered throughout the book with the telegraph poles, factory chimneys, trams, trains, etc. I've always been fascinated by the scene where Tintin goes to T'ai p'ing lu, to meet Didi. The very dark looking streets (no Siemens light bulbs here!) with the old houses and factory chimneys are strangely eerie. The old China and the new, so to speak.
One interesting point I might add is that the frame in question (frame 9, page 5 of Lotus) was originally slightly shorter, i.e. cropped right after the picture of the light-bulb. The writing on the poster originally just had the first three characters 西門å (Siemens electrical?) The frame was enlarged when Hergé adapted it for the 62-page colour format (sometime in the 40s, I think), and the other Chinese characters were added - 電機厰 (which I'm guessing means manufacturing plant!?)
Edit: See
here for the original frame.
One thing I've always wondered; who was it that drew the 'new' Chinese characters on that frame and throughout the book when Hergé adapted it, since Hergé had long lost touch with Chang by then? Perhaps that ought to be for another thread.