The chances are high that it's a 6.8volt zener.
Can you deduce anything from the associated circuit components? For instance it looks as though the three cathodes are connected together; is that point ground? Do the working ones have ~6.8v across them? Or are they perhaps across signal input lines as transient suppression?
Finally, (though should be firstly), have you tried calling the manufacturer? I've sometimes found specialist manufacturers quite helpful with technical queries.
