I actually worked on a computer that had the little magenetic doughnuts. Yes I am old.

I imagine the quality control at the factories had to be high. They were tiny and uniform. I believe they had 3 wires going through each, a read, a write (high or "1"), an unwrite (low or "0").

Each magnet represented a bit. Putting a lot of magnets together can cause problems, hysterisis for example, so to answer your question I believe the magnets weren't the erector set variety.