When I used to label connectors, I just used a sharpie (felt tip marker, popular in the US, but not sure about the rest of the world) on the edge of the connector. When reinstalling, a little alcohol on a rag wipes it right off.

"Sharpie" brand markers are sold in Australia but a more popular brand is "Nikko" - that's all I do as well - I use a Nikko to write on the edge of the connector "P112" etc.