As mentioned in #Bipolar Discrete UART I am in awe when watching this fabulous build :
The most interesting part is at 15:50 and I have examined and even simulated this circuit at An even better SCR-based sequencer. This is an impressive design that inspires me again because I want to turn it into a Johnson counter, which is a special case of shift register, which is also something needed for the #Bipolar Discrete UART ...
The name of the game is minimalism ! As noted before in 25. Germanium is not for ever..., the number of available vintage parts has dramatically shrunk and their price has exploded, so the fewer parts, the better !
The design that Leo chose has 1P1N per count: that's 20 transistors per digit, where a Johnson counter would use only 1P1N×5=10 total, which would use only 48 transistors for H:M:S.
Transforming the SCR latch sequencer into a shift register is not trivial but it should be possible. This would be also a great thing for the UART, because there would be no "slave latch". Looking at the schematic/diagram, which looks like a transistorized Dekatron, the clock pulse is driving ALL SCR through one shared diode, and "current steering" (something similar to the effect of a differential pair) will send the most current to one of the branches, which is pre-selected by the charge of a capacitor.
This is this individual branch that must be tweaked, adapted, transformed, so it behaves like a D-flip-flop. Then it will be useful for a wide range of circuits :-)
I know I should look more carefully at the works on the Shockley diode. More early SCR research can be found at https://www.rfcafe.com/references/electronics-world/transistors-negative-resistance-electronics-world-june-1969.htm