Transistors

[Mark Hennessy]

There's also an alternative, and rather more plausible version of the story:

From http://www.markhennessy.co.uk/articles/v...lass_types

Also, should the paint flake off, the transistor will behave to some extent as a phototransistor - plenty of unsuspecting engineers wasted hours searching for the source of mysterious humming before realising that their bench lights were the cause! Incidentally, there was an official phototransistor available - the OCP71 - which was filled with a clear silicon grease to allow maximum light transfer. The regular transistors were treated to a more opaque filling (as seen above) - apparently, if you scrape off the paint from a transistor to find a clear filling, it was probably a rejected phototransistor that was tested to see if it could be sold as a regular OC-seried transistor (reference).

Forgive the self-quoting, but the link referenced is a good one