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Comment by jsnell | Hacker News Reader

I would imagine they wanted 6 identical modules just for the manufacturing efficiency.

That still leaves the question of why 12 lights instead of 10. I would bet that the separator colons are actually produced by a separate mask+lamp, rather than having different masks for each digit on those two modules. I.e. to show "8:" you light up both the "8" lamp and the ":" lamp at the same time. That gets us up to 11. No theory on the 12th one though.

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Suspect it's just that the devices are a generic component, the application for projecting digits is just one possible application of a projection indicator. The physical constraints of the design require a rectangular projection array. 12 projectors, a 3x4 array, fits in a reasonable aspect ratio area. A 3x3 array would not suffice for this application, a 4x4 array would be overkill, and a 5x2 array, while optimal, would not fit well in the space available.

One for each . In the colon?

Almost certainly this. In one of the links by another commenter here, you can see a voltmeter where the guy changes the decimal. The dots are bizarrely high to be a decimal point, but right where you'd expect it if it's the bottom dot on the colon.

I didn't find that video, but several of these seem to be British made. The decimal point in British use was traditionally centred: 3·141.

(I still write it like that, in handwriting.)

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