alt Hacker NewsDenshi blocks:
https://en.wikipedia.org/wiki/Denshi_block
Still got mine, which has a synthesizer module that can be used for interesting experiments.
In Japan the company made professional versions for students - they were a bit bigger and more robust, but the same basic concept.
Not the one I had in mind, but very similar and definitely much cooler looking. The kits like these are still made and sold in various countries.