Stacked components printed onto small PC board interconnected on their edges.
The top thingee is where a tube plugged in (transistors were very expensive in the early days costing around $50 each) so tube were still used for a long time
Some consumer projects (radios etc) used modules like these for fairly standard circuit configurations (like amplifiers or voltage regulators)
Seach for "Project Tinkertoy" on the Web
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