Wow, this is great! IEE Clock Tube Time writes -
This one actually does have filaments. There are 12 light bulbs per digit; one for each numeral, and two for decimal points (left and right). These are neat little display modules that were made by a company called IEE (Industrial Electronic Engineers) way back before 7-segment LEDs were invented. Each light bulb sits behind a clear printed slide with the corresponding number printed on it in negative (the number is clear and the background is black), and in front of the slide is an array of tiny lenses. When a light bulb turns on, it projects an image of the number through the lens and onto the back of the lightly frosted plastic at the front of the display module. The Vintage Technology Association has a great exploded view so you can get a better idea of how this works.
The clock itself is fairly pedestrian although this design uses a quadrature encoder (the black knob on the upper right) to set the time. Instead of pushing on a button and waiting while the numbers slowly tick by, it’s much easier to just spin a knob.
Keeping time is the function of a DS3231 IC. The display is not multiplexed. A multiplexed display would involve a lot of diodes which would dissipate quite a bit of heat, and for this design, I use 6 ULN2003 driver ICs connected to 2 MAX7300 GPIO expanders. Technically I used devices that are pin compatible with the ULN2003 since nobody seemed to have any in stock. The microprocessor is a PIC18F2420 which communicates with the DS3231 and both MAX7300 devices using I2C.