You might get more help by linking this topic to a specific game. See "add game" link above on the RH side. At the very least, what is the game manufacturer and era?

Also, have you searched the Pinwiki for this?

Not Pinside... Pinwiki...

https://www.pinwiki.com/wiki/index.php/Williams_WPC

A & K are the leads for the transmitter LED, and E & C are for the receiver.

The opto on the left clearly shows the schematic symbol for the LED in the position for A & K, and symbol for the phototransistor in the position for E & C.

Unfortunately, I'm not sure if the S & E on the new optos are a manufacturer's mark or are supposed to indicate orientation.

However, if I had to guess, I might say the white dot indicated the electrical source (leg A on the LED for Anode). So, I would try orienting the slot opto where the dot matches the A position on the PCB.

But, since that's just a guess, someone else may know for sure.

Additionally, an LED transmitter should test as a diode when using a multimeter. However, I'm not exactly sure what the results would be for the phototransistor.

Do a diode test on the pair of legs close to each other. The side that tests like a diode is the IR LED transmitter (AK). The other side is the phototransistor (CE).