In 196x and 197x mainframe and minicomputers worked with ferrite memory.
One bit was represented by a very small ferrite ring.
On our PCB there are 16 * 16 = 256 rings in one rectangle.
The whole PCB consists of 16 rectangles which is an amount of 4Kb = 512B. (b=bit, B=byte, k=1000*, K=1024*)
Three wires go through each ferrite ring. The IC's are 7524 - read amplifiers.
It was an expensive part of hardware. I don't know, how it was manufactured.
Pictures:
Photo of a piece of a spare parts PCB and a split of the ferrite ring matrix, enlarged 40 times by an USB microscope.
