In camera maintenance we still have a box of 7586 nuvistors
that the PC-70 plumbicon preamps used.There were 2 7586's
per preamp board that was mounted right on the plumbicon
yoke assy.Most of the nuvistors in that box are pulls.I was
told that the nuvistors were changed during routine
maintenance even if they were not bad.So instead of
throwing out the pulls,someone saved them and mixed them in
with the new ones
Regards,
Swanson