Given this ID printed on the side of an old OPB815 optical interrupter switch:
And this pinout diagram from a randomly chosen datasheet for that part number:
One might reasonably be led to believe that the white dot on the part marks the LED anode. That’s what I thought, too, but the innards are actually rotated 180° from the picture: the dot marks the transistor collector.
Took me a while to figure that out; I eventually tore one apart and used my pocket camera to look for the blue-white glare of the IR emitter.
After the dust settled, I rummaged around in the impacted shitpile holding my paper documents and found the original 1982 datasheet, with my very own scrawled notes:
Back in the day, the dot on pin 1 marked the transistor collector…
Memo to Self: No, scanning all that old paper wouldn’t help.