Electronic – Unidentified IC with square marking

The question has insufficient information for a definitive answer, a photo would help.

Going by the description of it being a 24-pin device, and that it was socketed (which implies DIP, and also suggests replacement being a maintenance option), it could be this NEC ROM device: nMOS μPD2364, 64 kilobit, 5 Volt TTL ROM, programmed at fabrication by the manufacturer.

I'm going to post this as an answer as I've seen the same type of question come up multiple times, and for each I've basically commented the same thing:

It's pretty much impossible to identify this sort of component unless you either have a BOM of the circuit, schematic, or already know what they are.

If you have some rough idea of the function they perform and it is something more complex than say a discreet transistor, then you may be able to identify them by searching for parts with that functionality on manufacturer websites and comparing marking codes from datasheets. But really this is a shot in the dark.

Having said that, based on your description, the SOT23 package device is almost certainly some sort of TVS device - used for ESD and Transient (EFT) protection on inputs. These are quite common on RS485 devices as per this TI application note. There are components like this specifically for RS485 in the same package like this one. Doesn't have the same marking code, but will like have the same function.

The other package as pointed out by @jp314 in the comments is likely some kind of line filter. As you have found WE has some in a similar package with similar markings, so this is quite clearly what it is. In fact looking at the datasheets from that link, this part has a marking of 500 and looks identical to your picture.

For both of these parts, it could be inferred from the circuit what the parts do. Finding a datasheet for the exact part is the hard bit.