I imagine the fault would have rapidly led to thermal runaway in the OC81s and I assume this factory fault led to its demise.
When you acquired the set were the OC81s fitted with clip-on heatsinks? If not overheating could be the cause of thermal runaway leaving one transistor damaged.
There's really no need for matched pairs nowadays given the current scarcity of germanium devices - in the early days matched output transistors were basically a sales gimmick but in reality any difference in gain of the output devices which could result in audible distortion is countered by negative feedback*. I've fitted thousands of pairs of output transistors over the past five decades and I've never heard any improvement in quality using matched pairs in yer actual bog-standard transistor set. You need test equipment to tell the difference in audio quality between matched and non-matched output transistors. If you can't actually hear it, it don't matter.
*Note for pendants - I'm speaking generally, not just in this case.