I have a chain from 1994, I measured it and is still good. I broke it today. So searching the internet, I found the following information it is a Sachs perhaps GTS. I am guessing it broke at the "power link". Both ends are the inside plates, so whatever broke is completely gone.
Searching on lines I found "power links" but it said not to use pre 1998. Any suggestions for a repair? It is a 21 speed, MTB.