The 7 segment LED display module plugs into the main logic PCB via gold-plated pin mounted on the main logic PCB. This particular display is also badly corroded by battery leaks. The printed circuit wiring are silver plated. Each digit is actually very tiny and consists of 7 LED segments each lit up individually to form a digit, hence the magnifying lens built over the digits. 
Note that the LED digits silicon chip are directly bonded to the printed circuit board. If you look carefully you can see tiny gold wires leading out from the chips to the printed circuit wiring.
Note the size of each LED digit compared to a refill from a ball point pen.
The ability to make a LED display from such small dimensions is an engineering marvel. Because of its small size each individual segment needs to be carefully lit with the righ amount of brightness so as not to overhelm the rest of the segments. On hindsight if HP is to make the HP-21 again they would have better display technology to work with now, like OLED. In fact later generations HP calculators use LCD display which can have larger digits.
Note that the LED digits silicon chip are directly bonded to the printed circuit board. If you look carefully you can see tiny gold wires leading out from the chips to the printed circuit wiring.
Note the size of each LED digit compared to a refill from a ball point pen.
The ability to make a LED display from such small dimensions is an engineering marvel. Because of its small size each individual segment needs to be carefully lit with the righ amount of brightness so as not to overhelm the rest of the segments. On hindsight if HP is to make the HP-21 again they would have better display technology to work with now, like OLED. In fact later generations HP calculators use LCD display which can have larger digits.
The gaps where the plastic magnifying lens meets the PCB are not sealed. This probably allows corrosive substances to seeps in and eat away almost all the material in the LED chips. However sealing the gap will probably drive up the manufacturing complexity and cost and even turn around time. It could be also no easy manufacturing solution can be found which would seal the gap. Although the same resin use to coat the logic PCB may used.This this the foil side of the LED display module. Note the Made In Singapore imprint. Back in the 1960s to 1970s Singapore is in an early industrialisation and manufacturing cost are low there. HP has long since move manufacturing out of Singapore.
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