Oh, those things. I used to use UNIVAC 1108 computers, which had the system clock on the console with those displays.[1] It took about a hundred small printed circuit cards to drive the thing. It actually was the system's time of day clock, read by the computer, not a remote display of a counter elsewhere. If the muffin fans under the console were not regularly cleaned, the clock would make intermittent errors, which caused the operating system to have problems. I once had to write software to detect this.
"Which, for a clock showing HH:MM:SS, means 72 light bulbs to replace."
??? There are only ten digits, and you don't even need all of them everywhere. By my reckoning, you would need 3+10+6+10+6+10=45.
If you add the ability to display dd-mm-yy, you till would only need 3+10+6+10+10+10=49. I wonder what they did with the remainder. Spelling out the word "ALARM!"?
Or did they simply ship spare bulbs inside the product?
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.
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.
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.
These displays were made with many different mask options, including symbols and letters. I have a set where one mask is red (to give an optional red background to the displayed digit). For metering applications there were + and - masks. Etc.
When I was a kid, my dad would sometimes bring home scrap electronic equipment from his lab and I'd have fun taking it apart. I remember one of those displays, with the arrays of tiny bulbs.
http://www.decadecounter.com/vta/tubepage.php?item=10
http://www.decadecounter.com/vta/articleview.php?item=989
One company that makes them is still around:
http://www.ieeinc.com/about-us
A lot of other interesting old display technologies here:
http://www.electricstuff.co.uk/count.html