1.26-1.71 K under ambient light in the living room (no idea what angstrom I'm working with) and 160-200K after 5 seconds of cover, which puts it right in line with the Clairex CL505L that is the heart of the early T4 optos.
13th week of 1969 would be my guess. With the octet I concluded hastily that, should all fall within spec I can roll four T4 clones; I'm now leaning on slapping a range similar modern CdS cell to drive the meter, since that leg is out of the audio path and there is no benefit to a period correct part there. I tend to ignore what the meter is doing anyway..
Now to source the electroluminescent part, I'm pretty sure I can find a 2.2 meg resistor in a bin. There's also the matter of enclosure, but I do have some things I can cannibalize.