Ah, the joys of (inter alia) the OA47. You bring back memories.
A lot of those old components were light sensitive. Then Mullard realised that the OC71 was being used instead of the very expensive OCP71 and started putting a blob of opaque silicone in the OC71 cases. And then people discovered that the blob was too small, and a few minutes in the centrifuge would restore the light sensitivity.