I've come a step further. After spending some hours on Google Pictures I found this Indian company that produces for tractors and buses (no jokes please...)
Link to Hi-Lux
I haven't measured yet, but I'm sure I can find one that fits. But I can't see any dealers anywhere, I'm not sure about sending money to India (if they'll ship to me). Would there be an equivalent European or US based company producing these simple square old-school lamps? Or is any of you going by India shortly?
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